I'll give you the MCU unraveling in every direction being a good reason average audiences are going to skip Ant Man 3 and the like ... but where else is this really a problem? Harry Potter was an international phenomenon. Star Wars is Disney Universe big at this point. Everyone, EVERYONE knows the major DC characters.
Where is this a problem outside of the MCU? Where is this a problem outside of missing a few inside jokes in a few superhero films?
I'm not really buying this at all.
It's not Marvel, but the direction. Most studios now aren't just aiming for sequels, but cinematic universes. It's no longer like Jaws 1 to 2. It has to be Jaws Mother Edition, followed by Jaws and the Puppers. Where they are apart of an expanding universe and you need all of them at times to get everything.
Looking over this summer, but films either in one or attempting to build one.
Fast and the Furious
Guardians of the Galaxy
King Arthur
Pirates
WW
The Mummy
Transformers 5
Similar to Television where the stand alone episode is moving aside for series that have a continual story throughout seasons ala Walking Dead, GOT, etc. There is room for both still like films, but the shift definitely happened with audiences. They like the build up. And I could easily see where someone wanting to start in the middle, could be lost. If I did not know The Mummy, for example, would be CU, I'd be lost as to fuck why what happened in it(Still lost anyways and it was done poorly). Basically, every studio now wants to be MCU without putting in the work, but even MCU is getting harder to follow. With each entry becoming more reliant on previous work, nods, and setting it up that way. Black Panther could be hit with this hard, since they fully introduced him and have him a story arc in a film not of his own. So are they going to reintroduce him? What happened? Or just glance by it like you're expected to know. Or now they are saying Parker was some kid in IM2.... I don't even remember IM2.
TLDR
-Outside of MCU, every studio is just rushing into getting a piece of the pie. Hence why it's been terrible nearly everywhere else.
-With MCU, I do feel like they are even stretching me with the references, set ups to other films, and call backs. Where I'm at the point where a lot of it is starting to blend together. For me, it's a bad thing since it's become more routine. Still excited, but 3 of them a year is a lot to take in.