There’s obviously design influence here.
If you made every reply prompt people with a forced choice of new-topic vs same-topic reply, you’d indeed reduce engagement but there’s no doubt the function would get used more.
Hide the function, almost nobody uses it.
So, there’s trade-offs. This isn’t all-or-nothing. The aim is to reach all the users who would bother and currently don’t know and can’t even readily understand when someone tells them the feature exists. What portion is that? Definitely non-zero. Probably varies amongst different communities. It might be very high in a niche community of smaller numbers of highly-engaged users and much lower in a drive-by-commenting style place with less engagement and larger audience.
The status quo reduces use even for a communities of all A+ students.
Some communities will have reasons to really prefer on-topic focus and no problem with lots of small topics. Others may never care. Don’t make it hard for communities that want the feature. It’s a great feature.
At just a tiny push, if there were a built-in way to post something like the mockup above, it would at least make it easier for moderators to choose to easily show users the feature.
I don’t like the suggestion that handling this should all be about hands-on moderation rather than teaching and empowering users. Keep in mind that there are instances of pretty hands-off (and even relatively ignorant) moderators where engaged users still want to help each other have the best experience.