(Tongue in cheek): this is all my fault, ask ChrisF ...
Really, I do try to stop bad migrations, so do the rest of the mods. We just don't get to them in time in some cases.
I don't think bringing this up on Meta SO again is going to help much. While some bad migrations are happening, we're stopping a large number of them. Take thisthis for example, where I was on the fence and wanted to check with the mods before sending it over.
Chris is spot on with leaving comments indicating doubt that the question would be well received here. That leaves breadcrumbs for our users, as well as us [the moderators].
The people that vote against bad migrations (or flag us to stop them) are the people that keep up on meta. I think what needs to be addressed are the people that don't keep up with meta, and comments would work quite well with that. Feel free to also follow up with an 'other' flag to alert us that you feel the question would not be a good fit.
Finally, Anna having a diamond on both sites has really helped. Still, even with all of that in place, a few are going to get shipped over that really should have been closed. I do concur with the number going up a bit recently, so we [as moderators] will try to be sure to leave comments when blocking bad migrations.