I don't agree that was the situation by 2014 and earlier this year. I think by then the chain of dependencies was starting to form that was making things increasingly nasty.
a) There were loud objections to systemd which would normally cause Debian to back off
b) The dependencies were making it clear that Jessie was the last Debian distribution that not tying onself to systemd and remaining a mainstream distribution would be viable for. So the question was really switch now or switch in 2.5 years when the switch would be even more painful.
I do agree with you on the interfaces last longer than cold post BTW. I think that while systemd is a huge plus. Replacing systemd say 20 years from now will be very difficult. Essentially the modules will each need to be reimplemented in a way that's backwards compatible, offers what the future features are and allows partial implementation. The kinds of problems say Microsoft, Apple, DEC or Sun had in pushing forward. I'm a fan of tight vertical integration but there certainly are counter arguments against it and for loose coupling.