but none of those were legitimately bad movies that don't deserve to have sequels

Ok, but how is that relevant? I'm trying to argue "necessary v unnecessary sequels" has no (well really very little) relation to actual quality of the sequels or original films. You're making a counter argument against points I'm not making. Ironically I am very much in favor of "unnecessary sequels." They're often not nearly as good as the first film but they're still fun and often have a grander scope (e.g. Pirates sequels)

sheev [emperor Palapine] in it?

No. At the start of the film you hear Tarkin tell us "the emperor has dissolved the senate and now regional governers will keep the systems in line." The emperor is in no way set up as the big bad. Tarkin is the embodiment of the empire even if there isn't dead emperor at the end of the film. If the emperor had a role in 4 akin to his role in 5 (aka Snoke's role in TFA) then it clearly sets up a sequel.

I think, without any other films, Obi-Wan guiding Luke to "use the force" and destroy the death star would have been considered a satisfactory coda to the jedi stuff if no other film had been produced. It's a decently compelling counterargument though.