In the last 90 days, 73 outgoing migrations have occurred, in the following distribution (we'll skip the Meta migrations, which are 18 in number). 37 to your open migration routes, and 33 through the moderator channels.
- 11 to Cross Validated (3 rejected)
- 11 to Physics (6 rejected)
- 7 to Stack Overflow (1 rejected)
- 6 to Mathematica
- 6 to Math Overflow (1 rejected)
- 6 to Academia (1 rejected)
- 3 to Signal Processing
- 3 to Computer Science
- 2 to Computational Science
Putting aside that Physics has an over 50% rejection rate, the rest are too few in number anyway. Migrations are not enabled based on logical bridges - we enable migration paths for a site when the site receives a high frequency of off-topic questions that belong on that site - enough that moreso it's a burden on the moderators to handle the migration via flags. And also that the migration rate is actually accurate with little rejection rate. Sometimes it's simply a matter that in the long run, compared to the volume that the site gets (and Math gets a very nice question income), there actually isn't all that much off-topic for the site, nevermind that belongs on another site.
At an average of less than two migrations per month on any of these, I'm not inclined to agree with opening up additional migration routes. Mathematica I still have been keeping an eye on for the past few years, but the situation doesn't seem to have shifted enough to warrant opening the gates to there.