it sounds like a Linux password is a red herring, and a secure password even more so

Yes and no. A secure password is extremely important against some security threats, but completely useless against others. It’s like vitamin C. If you don’t get enough, that’s a massive problem and opens you up to a ton of serious issues, same as if you don’t have enough complexity in your password. But even if you do, it won’t effectively protect you from, say, cancer or unprivileged malware respectively.

There’s nothing stopping any program from attempting to bruteforce your Linux password, literally running through possibilities hoping to guess it. Modern password implementations usually have some form of bruteforce protection. If you’ve ever entered your password wrong in sudo or KDE’s lock screen, it usually hangs for a few seconds before telling you your password is wrong, even though any modern computer will have determined it was wrong in literally an instant. This is to prevent a malicious program from endlessly trying random guesses until it gets it by making the time it would take to guess a sufficiently unique password too long to be practical. Your phone and optional software available for Linux go a step further, imposing longer and longer delays with each subsequent failed password attempt, and also prevents malicious programs from spawning many threads each independently calling sudo to bruteforce in parallel by completely disabling access until the time penalty elapses. Though you absolutely do need a sufficiently secure password, making it overly long has diminishing returns past a certain point, it doesn’t matter how many millions of years it would take to bruteforce with a 1 second delay for wrong attempts, but the upgrade from millions of seconds with a simple password like “hunter2” to years is the important part.

Also, a password with no encryption is like a padlock on a wooden box. Even if they don’t have the key, they can still just cut the box open. In computer terms this would be like if someone accessed the files in your SSD directly and injected malware with root privileges, since both completely bypass the check that’s “normally” supposed to stop unauthorized users. Encryption can help but like you said, physical access is generally considered game over anyway unless they found your computer while it’s off and it is never returned to you for you to enter your password. A computer with encrypted everything wouldn’t be able to boot. Your EFI partition and especially your BIOS/firmware have to be unencrypted, and anything unencrypted can be tampered with by a sufficiently skilled attacker with physical access to add things like keyloggers and backdoors that sit dormant until you graciously decrypt everything for them.

Your password strength matters a lot more with encryption though. If you’re going to the trouble of full disk encrypting your computer, make the password as long and random as you can practically remember. If someone is trying to decrypt your computer’s drive, they’ve probably imaged it and are using a separate machine with no rate limiting whatsoever, and modern GPUs can do a ton of cryptographic operations in a short time. And don’t use that password for your user account once decrypted.

If SSH is disabled the class of attacks to be prevented are users ‘voluntarily’ running malware pretending to be goodware.

More or less as far as I know, provided you don’t have any other way of remote access (VNC, RDP, Anydesk/Teamviewer and similar, that weird Steam remote desktop app, a server running vulnerable software on an open port that can be hijacked, etc). In computing, the general rule to follow is if you don’t need it, don’t enable it, otherwise it’s ripe for abuse. That being said, your router should be configured to block local port access from the internet anyway, but if you have another infected device on your network, that’s a major threat. If you do want SSH, configure it to only accept the keys of your trusted devices and not just respond with a password prompt to any device that comes knocking.

True, but does anyone operate this way? At that point it becomes an iPad or a Chromebook.

“Trust” in computing is fickle and complicated, just like real life. At the end of the day, you have to make a decison on who and what you personally trust. An iPad or Chromebook would be the least trustworthy computers in my mind because they’re locked down and administered by companies I absolutely do not trust, and though the locked down architecture does prevent other malware from infecting it, there’s probably already malware by any other name on it with proper Google or Apple security signatures that came with the device from the factory.

This is the same as if your distro maintainer is untrustworthy. They could slip in malware into the official package manager or installer ISO and you’d never know. I personally trust a reputable Linux distro over the literal biggest tech corporations in the world, but I’m still putting my faith in an organization I do not control nor personally know the people in control.

Open source is more trustworthy than proprietary software because the source code is available, but even that isn’t completely guaranteed to stop malicious code from making it in. The recent xz backdoor comes to mind. You’re still trusting that the other people looking at the source code actually catch the malicious part, and that’s not guaranteed even with the most trustworthy people when everyone working on it are overworked, stressed, and in the grip of tunnel vision to get their small part of it done like software developers tend to be, and even when that happens, it might be months or years down the line after the damage has already been done. There’s a reason a full security audit of an app can cost anywhere from thousands to millions of dollars depending on how big the codebase is. Also, because the vast majority of software aren’t compiled in a reproducible way, you don’t really have a guarantee that the actual binary executable that’s on your computer exactly matches the source code unless you go through the (usually difficult and frustrating) process of actually compiling it yourself. Sure, you can probably assume that the official binary released by the source code authors and signed with their cryptographic keys matches the source code since both come from the same place, but that’s not guaranteed and you’re still trusting a person or organization.

But wait, there’s more! The compiler you use is itself a program that needed to be compiled by another compiler, and so on and so fourth until you literally reach the stage decades back when someone manually wrote the individual bits for the very first compiler in that chain. A malicious compiler can be made to obfuscate the fact that it’s malicious, and only a manual review and reverse engineering of the raw binary (without reverse engineering software, mind you) can prove or disprove it’s compromised.

Finally, there’s hardware. Even if you audit every single literal bit of software, the processor itself has immense complexity that you can’t audit without, 1, extremely expensive scientific equipment, and 2, destroying it in the process, and that’s only one chip out of the tens of chips in a computer. Your processor could have secret instructions that bypass all security and your only real hope is to bruteforce every possible input to see what happens. And proving existence of a backdoor is intrinsically much easier than proving absence.

I’m not trying to scare you, but I do want to illustrate just how hard it is to have absolute trust in any computer. At the end of the day, you can never have a computer you completely trust unless you manually assembeled it from raw materials (not aided by any existing computer) and hand wrote every bit that goes into it. Like I said, we all need to make a decision to have faith in some person or organization we do not know. You can spend every waking minute auditing every last part of your computer, hardware and software, but then you wouldn’t have time to actually use it for the things you want to do. There’s no solution to this, there’s only higher and lower degrees of trust and security, which only you can determine for yourself.

So no, no one operates that way, because it’s impossible.