Debian never gave guarantees for anything but their default init. That has always been like that, it is just the init that changed. How could a responsible distribution make claims that init systems it never made am effort to test is supported?
I think users are mostly happy (or blissfully ignorant about init systems) with systemd. If they were not, then users would storm devuan. That distribution has seen lots of press when it started, so people did know about what is happening there, yet interest does seem slow.
I also think that maintainers would not have gone for systemd if they did not think it had benefits for their users. Contrary to what you think maintainers do care for having people use their distribution. The fact that systemd had convinced developers did also factor into the maintainers decisions. So did advantages for the packagers: Getting rid of init scripts was a big part of that. There were lots of factors considered at Debian, check the CTTE discussion you liked to earlier for more.
I do not think it matters whether software depends on an init system. Software depends on other software all the time and will adapt once some better option comes along.
Actually I find it reassuring that things start to depend on systemd: It means that it is reasonably simple to interact with the system and that it provides something worth the effort to talk to it. Never seen that before on Linux.