I think a lot of these points are right, but I think a lot of the fans aren't necessarily just upset about increasing inclusion, since a lot of the fans who have become disappointed in the Star Wars trilogy enjoy other works that are plenty inclusive (I consider myself to be in that category, to an extent: the Expanse is to me what Star Wars used to be, a sci-fi universe I can count on enjoying fresh content from).

I think that part of the thing that's having issues is that Star Wars is having something of an identity crisis. It's not sure if it wants to be a kid's movie or not, which wasn't a problem with the Original Trilogy (which generally didn't try to be a kid's movie), wasn't a problem with the Prequels (which generally did, though Episode III was more mature: Episode III aged with its target audience), and now the New Trilogy alternates back and forth.

One of the things that made the first Star Wars hold up so well (and it doesn't actually hold up as well as fans say: I teach it so I'm pretty familiar with it) is that the story is very much traditional in format. You have a hero who faces real loss and accomplishes a real triumph by the end.

The Force Awakens pulled that off better than The Last Jedi, which is why you didn't see quite as much of a backlash against it as you saw with TLJ.

The thing that TLJ got in trouble with is that it presented a problem but not a solution to that problem, and it's unclear who the hero is in a plethora of characters and action that really reflects an approach more similar to the MCU.

I think that there are a lot of people who do use the focus on diversity in casting as a point to attack the newer films because they fail to see that The Last Jedi is really nothing more than a blunder. It's no worse than the Prequels, but they don't see that because they're looking for a reason.

While the more logical reason is that the Star Wars universe isn't a magic formula that guarantees a good movie (anyone familiar with the EU should be aware of the fact that not all Star Wars content over the years has been good by a long shot), and TLJ's team was drawn off in different directions for marketing concerns and in an attempt to swell the cast. I don't think the motive for this is necessarily solely because of diversity; I think they wanted to hit on the same success the MCU has had with its broad array of characters and open paths for more content down the road.

There are also real missteps, things that should've been caught. They literally had a plot point in TLJ centered on a traffic violation. That's a crappy Monday, not a Hero's Journey. Solo got a lot of things right and kept stakes high, and it worked really well with the new formula.

I think that a lot of the fandom had absolute trust in Star Wars, a belief that it could do no (significant) wrong. When they were left disappointed, they sought a scapegoat without considering the real reason to avoid accepting the fact that there could just be a mistake in how the franchise has been handled that doesn't revolve around some larger worldview.