After the C Series discussion got going again, I went back and did some math on the back of a napkin after looking at the UAL Mainline Fleet page. Management has said more than once 25 years is about the viable lifetime of an aircraft purchased new.
What I've realized is that swapping all those 700s for Max 9s and 10s makes a fair amount of sense (well, setting aside the wisdom of buying a fifty year old design). Our oldest 73Gs and 319s are about 20 years old (about 100 aircraft with 118-128 seats). They've got a fair amount of life left on them and thus there is not a rush to augment with more of the same (or C Series/E-2).
Meanwhile, our oldest 320s with 150 seats are actually 24 years old. Of 97 frames, about 40 are over twenty years old. Of 56 757-200s - 24 are older than 22 years (a few are in the neighborhood of thirtysomething). Of our 35 767-300s - 21 are over 24 years old. Our 777-200s are getting up there too, out of 19 aircraft 13 are over twenty years old and our oldest 777-200ERs are starting to turn 20 now as well (a dozen as of this year).
So, in the next five years or so there is definitely a more pressing need to replace larger mainline aircraft than add smaller ones. At the same time we don't have a lot more gate space coming soon at our most congested hubs (EWR/SFO/ORD/LAX). So up-gauging from 320s to slightly bigger aircraft (800s/900ERs/MAX, etc) is a way to ultimately replace older aircraft while increasing capacity within our existing airspace/infrastructure constraints (and avoid growing pilot numbers too much as well I'm afraid, ">15k" pilots? Not.). The five more 798-9s we have coming in the next six months plus the 14 787-10s we have arriving starting next year will allow our oldest 756s and 777s to continue being moved from international to domestic p.s. type service where they can receive more care and feeding from maintenance... or be parted out if the economy tanks.
I think this general trend of removing our oldest larger jets from international services will continue until a MoM aircraft enters service. I think the MoM will be used primarily for transcons and TATL services, allowing the hodgepodge of geriatric jets being used for p.s. service to be retired en masse.
As for the small end of our fleet, I'm not holding my breath. Previous management seemed much more disposed to buy a NSNB. Kirby's maneuvering so far has clearly been aimed at maximizing what he can do with RJs under current scope and it appears we are set for a while. I pre-assume we won't give an inch on scope when our contract talks open. Once Kirby has run out of ideas he might then bite the bullet and buy a NSNB, but he has a few years before he absolutely has to chuck a rock (long enough to get a new contract signed - anyone want to guess what the MAX-10 will pay?).
If anyone has better insight or fill ins - I am all ears.