This has been a very interesting thread! My thoughts for whatever they are worth:
1) I don't think the heat is an issue because it is hotter. I think the theory about pulling kids out of school more often makes sense though.
2) I think moving former
DAS users to SB makes a BIG difference since they can only wait in one line at a time and LL lines are shorter, the SB lines move faster even with more people in them.
3) I think increased living costs overall are absolutely making a difference. Our insurance costs have tripled in 15 years without ever making a claim. Our electric costs have gone up 70%. For us we can still afford Disney but that's where the decrease in quality leads us to look at other places instead. And while we can't compare the quality of WDW food with Rome, we can compare it to WDW 10 years ago. Josh at EasyWdw (I miss him so much) was great at posting side by side pictures of offerings from year to year. The food got smaller, dryer, and plainer. And that's forgetting the price increases.
4) I do think people underestimate the impact becoming very political has affected how people feel about disney. Agree with them or not, I do not want any politics on my vacations. I tell my patients not to watch the news all the time because I've seen how it impacts their health. People need a break. Vacation is a break. Universal has been so much smarter about this it blows my mind.
5) I compare Disney to my legacy cell plan. Lately I shopped around and found out that I can't even get what I have with other carriers (or my own carrier if I switched plans) no matter how much I pay. It's just not offered anymore. With disney, I'd pay a big price increase if I could still get what I got in 2016 (late hours, early hours, better themed hotels, better customer service, better themed merch, classic food they don't have anymore) but it's not offered no matter how much we pay. I just don't get the feeling Disney likes its customers much.
We will go back, but I can't imagine taking a big chunk of time off work and making it our big annual family vacation again any time soon.