I'm not a crew resources guy but the md88s are slowly being phased out with several retiring in the next 3-5 years.

We currently have about 135 737s and still have another 60-70ish rolling off the assembly line. So mix of retire 88 and some ERs and growth of 73N fleet. Not sure of the planned #s but the category is growing and there is much rejoicing. I doubt a relatively new hire would have enough seniority to bid in to the category when it is first advertised this fall but I still don't think it'd take that long to get there. Msp is pretty senior so it won't move lightning fast like 73N NYC (plug to <60% in less than a year).

During walk around you can eat off the landing gear and wheel wells of the new 900s. 88s not so much...trust delta will squeeze every last drop of blood from them. They make the company a ton of money.