It is more than merely an "init manager". And I disagree that it is the best thing ever - it is perfectly possible to operate linux without systemd.

> but I have the feeling the huge upside is often overlooked.

It is fine to objectively compare trade-offs. However had, it has to be a fair comparison; we can not start with "init manager" because systemd does a lot more, so how can a comparison to any software with less code be fair then? runit doesn't do much more than for initializing.