Self perpetuating growth.
Systemd integrates many functions so people default using it and add even more functionality that bring even more people into ecosystem.
Which is basically how every tech ecosystem works.
The problem is that linux is supposed to not be just_another_centrally_controlled_ecosystem, so when systemd abuses their popularity by enforcing whole ecosystem (rather then cut itself into separate pieces), that is worrying
It seems like you're attributing to perceived malice ("enforcing") what is better explained by a combination of trying to get things done and not prioritizing replaceability higher than the benefits of integration. In other words, a project that simply has different priorities.
It turns out that many other people also like seeing stuff get done more so than they care whether the project in question is trivially replaceable.
Which is basically how every tech ecosystem works.
The problem is that linux is supposed to not be just_another_centrally_controlled_ecosystem, so when systemd abuses their popularity by enforcing whole ecosystem (rather then cut itself into separate pieces), that is worrying