But they usually aren't big babies making demands and throwing tantrums on those projects.
I'm pretty sure if some other controversial dev at some other FOSS project came out and said "I need money to continue developing" there would also be people "making demands and throwing tantrums", because people generally don't want to support assholes. The people throwing actual tantrums are people jumping on their high horse going "Oh, you don't want to support an asshole. What a freeloader you are."
Since you're arguing that we shouldn't take their politics into account tell me, would you defend them just as vehemently if they were fascists and ran a fascist instance? Or do you make that argument only because your own political views align with them? Because despite clearly different ideological views their actions and support aren't that far from fascists. Fascists peddle in propaganda, censorship and revisionism. lemmy.ml regularly does propaganda, revisionism and censorship (with one of the main devs being the one doing the censoring). Fascist need an outside group to direct anger at. Lemmy.ml has established "the west" as the outside group to be angry at. And obviously both have a clear affinity towards authoritarianism. We're only missing terror and cult of personality (And I'm sure this one isn't far off if it's not already present but not apparent). So would you defend them if they were fascists?
PHP is hardly doomed. It's still getting new releases (last one being 19 days ago) and it's still relatively popular for web development, which means it's going to be support for quite a while. The rest what you've said about PHP is more or less just your opinion. As for Rust, Rust is a good language but Rust isn't very popular at this moment and it's even less popular when it comes to web development because most of the things that make Rust great are things that are simply not that important for web development. Most web development languages are performant enough, rust being even more performant isn't going to be noticed. And because social media sites aren't exactly security-critical Rust being memory safe won't play a big role. The only clear benefit Rust has over PHP is being strongly typed.
But there's a very obvious benefit to PHP. You simply develop faster. If you had 2 identical web projects, one in PHP and one in Rust, and you were equally skilled in both languages you'd finish the PHP project faster because PHP is literally designed for web development. Speed is a much bigger factor when developing a Reddit style content aggregator site.
As for the second point. You have an issue with fragmentation, but your solution to Lemmy devs refusing to cooperate is fragmentation? People forking Lemmy will also lead to fragmentation. Fragmentation is bad, but eventually people will gravitate towards one solution and fragmentation gets solved. It there's very little different between the fragmentation happening between Lemmy forks or Lemmy and Mbin. IMO fragmentation is inevitable and I would rather see it happen right now rather than sit through N different Lemmy dramas until everyone gets so fed up with Lemmy devs that Lemmy gets forked and the fork becomes more popular than Lemmy.