Bonnie, I wouldn't call it hatred; your opinion is well founded and based on your knowledge of the details! Plus, I'm absolutely with you when you say that good followups should share the general themes of the movies that came before them, and that it's important to know when there's no more stories to tell within a certain universe.
That said, I do feel that MU needs to be defended. It does expand on a plot point that's in the first movie but never properly explored, always just taken for granted; namely, Sulley and Mike's friendship. There's also the theme of taking care of someone (Boo / Oozma Kappa) although they don't want to, and there's a breakup-makeup scenario that hugely impacts the way the plot develops. I'm in conflict about the Scare Games because they feel "tacked on" to me, if that makes sense, but overall, I can definitely identify certain patterns - for example, the training scenes, Mike going missing in both movies and Sulley looking for him, Mike stealing the key cards, etcetera. There's an extensive listing of "Call Forward" instances like that on the movie's TVTropes page
Plus, I believe they didn't get new personalities at all; in fact, IMO this is the true brilliance of MU - Pixar hit on shades of the characters (mainly Mike, Sulley, and Randall) that were always there but very
- Mike is really broody and negative in MU, but a chipper goofball and motor mouth in MI. Doesn't go together? In MU, he has no friends, struggles to get a job he loves, and all he basically wants is for someone to recognize and accept him. After Sulley does just that, his life takes a turn for the better - he has an awesome friend, a job he loves, a fulfilling romantic relationship, and generally no reason to complain. So why should he be negative anymore? But the signs of his past self are still there. For example, look at the commercial and magazine scenes - both Sulley and Celia are clearly worried that Mike is gonna slip into a bout of depression, and Sulley especially is really surprised and relieved when it doesn't happen. Plus, other parts of Mike's character stay largely the same, like his snarkiness and his tendency to rush into situations in what he believes is the right way.
- Sulley is really brazen and obnoxious in MU, but down-to-earth and humble in MI. Thing is, he basically tells us that the MU personality is all just a mask held in place to try and please people he cares about. ("I act scary, Mike, but most of the time... I'm terrified.") After Mike tells him to be himself, he accepts his actual personality, which he can when he realizes that that is one that's beloved by a lot of people, and his constant, slightly over-the-top humbleness may just be an attempt to make up for the idiocy of his past actions.
- Now Randall is where it gets really interesting. While the speed of the transformation in MU was probably too fast, the transformation itself makes total sense, especially in what this means for his relationship to Mike. Randall is shy, insecure, and kinda sweet in MU, but downright psycopathic and cruel in MI. Yet you can already see in MU that even back then, it's all just about him. Mike reassures him before the first Scaring class but never gets reassurances in return. Randall pressures him about stuff he wants to do and when Mike doesn't want to participate, Randall just leaves on his own. He doesn't even have any qualms about joining a group of people that have a habit of bullying his only friend, just to get popular. So of course in MI he'd be obsessing over how that little nobody he left for popularity has managed to get more popular than he is. Plus, he only ever directly attacks Mike, never Sulley. And look at Mike's behavior in the Scream Extractor scene - right up until the machine approaches him, he's convinced that someone who used to be his "chum" wouldn't actually harm him. There's loads of familiarity between the two in little gestures throughout the movie if you pay attention, while Sulley is rather indifferent to Randall except for being protective of Mike.
To be fair, it took me quite a while to see all this; when I left the theater after my first viewing, my mind was reeling trying to get it all reconciled. But it's all there. If you're interested in further elaboration, there's a huge listing of stuff like that on the movie's TVTropes Fridge Brilliance page
I agree with you on most of the rest, though. I did enjoy Cars 2, but it kinda fails its purpose of being a sequel. I'm rather indifferent about Finding Dory and Cars 3; while I do like the originals, they're not important to me the way the Toy Stories and Monsters movies are.
The two Toy Story sequels we know, on the other hand, are logical and beautiful expansions of the original story; in fact, I was always convinced a third part was still missing, so I was excited and not at all surprised when it was announced. And I feel hugely conflicted about Toy Story 4; I actually want
it to be largely different from the previous ones so as not to "tarnish" the utter perfection that is the trilogy.
Phew, sorry for the wall of text. But you asked.