Exactly the point. Since UA has dismantled the big advantage of being aligned with M+ for those connecting to UA for domestic USA travel, CM's only distinguishing factor is their mileage-based earning when compared to AV.

Delta would be a more consistent partner in terms of flight experience, and it would separate the two programs completely in the eyes of Latin Americans who never set foot in North America. For those pax, they are currently offering identical benefits.

I have long thought that the entry of AV into *A probably caused a lot of resentment at Copa, who has been a long time valuable partner in this region. While it was good for Gringos and Canadians, it probably made things a bit harder on Copa. Joining DL/ST would at least give different benefits to the passengers of the two competitors.

Whether or not those benefits are better would be an individual decision for each passenger. Keep in mind that Copa was a ST member prior to the merger decision by CO/UA, and they did just fine with that alliance.