one thing to bare in mind is that Plagueis was successful in his goal to have everlasting life. He figured out how to reverse his aging back to his prime and how to rapidly re-generate from injuries. He even figured out how to use the force to just instantly kill his enemies

It wouldn’t be too much of a stretch to believe he was expecting palps to kill him soon and would have some sort of contingency plan to go into hiding for awhile and gather up a power base to fight his old apprentice. And then waited for the empire his apprentice created to fall before trying to take over the galaxy with the power base he built up