I think the point is that you can't have two competing init systems: the market will eventually need to standardize on one of them or risk forking the entire Linux ecosystem. What you call "modern sysv" would look a lot like systemd depending on your definition of modern. I think that definition of what modern should look like is what's being debated here.

The big features sysvinit are missing require user space applications to be able to interact with [generic launch system]. Those applications must use some sort of standard taxonomy and flow control. As applications are ported to [generic launch system], they cease working on sysvinit systems because sysv either doesn't support those features or supports them in a different manner.

Sysvinit doesn't really do all that much, to be honest. So it's really easy for [generic init system] to emulate most of its functionality. [generic init system] does a lot more, so it's harder to copy. Even more, if you were to "clone" [generic init system], applications make big assumptions as to how [generic init system] functions, so your [generic init system] replacement would need to function nearly identically to [generic init system].

Basically, someone made a call and said "this is how a modern init system should function". There is a vocal group of people who disagree with that definition. But no matter what the definition is, people would disagree because different people have different ideas about what Linux is/should be. You would see the same problem if a sysvinit derivative was the leading candidate instead of systemd.