You wrote: I am about maximally biased on the question whether it is a good idea.

By this you meant that you are strongly in support of top-level tags on MO?

Or did you mean, as you mentioned to me in the past, that you are not really a big fan of migrations?

Or something completely else and I entirely misunderstood the parenthetical?

But I certainly agree that migrations from MO to MSE do not happen too often and migrations in these specific tags are not very likely. (At least K-theory tag is IMO a rather advanced topic, so it seems unlikely that a question in that tag would be considered too easy for MO and migrated from they to this site.)

Hm... now that I browsed the list of recently migrated question a bit, I see that migrations from MO to MSE are much more frequent that I thought.

Still, some of them might have been done by mods - there the tag compatibility does not matter.

And most of those migrations seem to be in the "less advanced" tags.