It's a giant mess of interconnected programs that could theoretically still be disentangled, but in practice never are. It was very quickly and exclusively adopted by pretty much every major distro in a short period of time, functionally killing off any alternatives despite a lot of people objecting. Also, its creator was already pretty divisive even before systemd, and the way systemd was adopted kinda turned that into a creepy hate cult targeted at him.
There's nothing actually wrong with systemd. I personally wish there was still more support for the alternatives though. Systemd does way more than I need it to, and I just enjoy having a computer that only does what I want.
Am not sure about "giant mess" but indeed it has a lot of moving parts. All that said, systemd is solving tangible problems which is why you will almost never see maintainers complain about it. It's mostly Linux users which by definition oppose any change, Firefox 4 → Firefox 5, Gnome2 → Gnome3, SysV → systemd
"Giant mess" was maybe an uncharitable phrase to use, but it really is a lot of programs that are always used together because trying to mix and match it with other stuff or even just take pieces out is a massive pain in the rear. Again, I don't actually object to systemd. I use it myself because it's so much better supported. It is not always ideal for everything though, and I'm a little sad about the lack of support for other options.
The idea that Linux users by definition oppose any change is just silly though. We almost all got here by making a big change in how we use computers. Almost any change will be opposed by at least some members of any group. That's just how people are. That's not a special thing about Linux users. Sometimes a change that is overall for the better causes some things to be lost, and saying the people who are unhappy with that "by definition oppose any change" is kinda creepy, if I'm being honest. In particular all of those examples you gave are times people were forced into a change that was not all for the better, especially in the short term, with little notice, and no opportunity to voice their concerns in a more constructive manner. Of course some people complained. It would be weird if they didn't.
this is a Just World fallacy, assuming the best thing will always be adopted and therefore everything not adopted is worse than [current thing], when it is entirely possible that there are in fact better options
This is not a Just World fallacy because I'm not talking about justice or people getting what they deserve. My assumption is that OS developers are competent. Until I see otherwise I'll maintain that assumption.
I didn't say there were better options. I didn't say it shouldn't have been adopted. I said it has some drawbacks, wasn't rolled out very well, and I miss having other options even if they aren't as generally useful for everyone, and it is inevitable that some people would complain because of that. That isn't a problem. It's okay to complain sometimes. We all do it.