Why should MX be likely to join OW before Alaska, Air Tahiti Nui, Hawaiian AIrlines, etc, which have been AA partners even longer? (But like MX, Alaska, for example, partners with airlines in other alliances too!)

Also, haven't you heard about Aer LIngus? It's partnering with AA despite (and because) it's leaving OW! That alone should make it obvious that partnerships and OW are two totally separate things!

And btw there are different levels of partnerships. MX is an "ordinary" AA partner, ie, simply "normal" miles earning and redemption. Alaska is a "better" AA partner (and the only non-OW one that I'm aware of at this moment) in that all flights on AS are elite-qualifying!!!