I don’t agree with everything your saying but it is important to remember Flash Gordon. Star Wars, like old space opera serials, is supposed to be a never ending saga, where we know no matter what the heroes will always be there to save the dar. If the evil can be defeated for forever, that’s it’s not really never ending is it?
This gets back to the fact that I fully believe that many people just fundamentally did not want to see movies made set after ROTJ, whether they say so or not. Simply put, to make a story set after ROTJ, you needed to undo that ‘happily ever after’ victory. What made TFA and TLJ so great is that they didn’t just wantonly undo it, they gave a thematic reason for doing so that justified their addition to the story. TROS… not so much.
But Flash Gordon didn’t go on forever. There were three Flash Gordon serials, with each episode within each serial only being 12-15 minutes long. Even the OT is split up into “episodes,” each film is equal in story to one full Flash Gordon serial. Not to mention, there was a two year wait in between the film serials, not unlike the OT. However, in the ST’s case, there’s been a 30+ year wait. So having the same villain as all the others feels much different from 30 years ago is different than just bringing back the same one two years later.