Does it cost money? If the answer is yes then ....

A lot of people are saying this is happening

Oh it’s well under way.

3 737-400s and 12 737-800s

That’s a lot maybe we are losing the 700s and everyone is getting displaced haha

Before anyone gets to excited I would check United’s and Americans scope clauses. I believe the UAL agreement mirrors Delta where this type of flying would not be permitted by a connection carrier if that carrier or its parent company owns or has operational control of the airline flying non permitted aircraft even if all cargo.

I'm sure Mesa has lawyers to handle these issues.

If I was a mainline partner, I would be much more concerned about losing Mesa as a provider. Cargo is more lucrative, don't need FA's, cut your training costs in half, no part 117, so 30% less pilots needed. Mesa would have more leverage to tell a mainline to frig off. Mesa would have zero issues finding pilots to fly a 737, and could be far more picky about things if they wanted to be.
Unfortunately, this will further remove any bargaining chips the pilot group has. Be careful what you wish for ;)

I’d quit Mesa before I go fly old beater 737’s for this place.

I seriously doubt Mesa will have trouble crewing the 737 fleet.
Rational "I just do it for the type" "just until WN calls" "it'll look good on my CV" etc.