It doesn't matter what each agreement says. Delta will come to these companies and renegotiate their agreement for 76 seaters with a higher payment per block hour for the regional. Most regionals looking at the massive training costs associated with HR3371, increasing maintenance costs with erj 145's and crj 200's, and the hope of a better profit margin will make them sign whatever Delta is offering.

Even if your airline has X amount of airplanes until 2050 it doesn't matter. Everything is up for negotiation and Delta is pretty good at shuffling flying around.