Honestly?

In what political reality could ALPA demand the elimination of Regionals.

The best option on the table is a slow transfer of seats from regionals to mainline. Asking for this to get done in a single contract is impossible. DAL was an improvement and the UAL TA scope carries the torch one step closer to the day when all flying is done at the mainline. The ALPA negotiators were not stupid, they knew full well that scope was THE single most important issue facing pilots today, and the scope clause reflects that understanding.