This is the nuclear option. If all else fails, it will be the solution.
That said, I'd like to try a few other things first, and see how they work out.
First off, we have a gigantic backlog of questions with pending close votes on them. Many of these may need to be closed, but few should be migrated. A recent change to how migration works removes that possibility for about 8.5 thousand questions with outstanding votes - that's a pretty sizable chunk.
Second, I've implemented this suggestion:
This is fairly subtle - but it's worth a shot, since a reoccurring complaint is that folks on SO don't understand the scope of this site. I'll be watching to see if it has any effect.
The % of worthwhile migrations does seem to be fast-diminishing, but there are still a number of questions sent over from SO that are not only accepted here, but do fairly well - up-voted, answered. If this becomes such a small portion as to be expendable, then so be it.