Downgrade? I have had 4 trucks in the new generation that last being a 2017 Yukon XL Denali that I bought brand new. The new trucks suck. Quality is terrible, and I have had issues with everyone of them. I am a maintenance freak and really take care of my vehicles. I could go on and on with a huge list of warranty issues with all 4 trucks but I will end with saying I hated them for the price they were. Quality was not there. I am still mad at myself for doing it 4 times!!! Money was not an issue for me and I went "backwards" to the GMT-900. A much better ride in my opinion. Ride, seats, comfort my "new" 2013 wins.
The new trucks have much better fuel mileage and the 6.2 with the 8 speed Denali is crazy fast and tows like a boss! Safety features were great. I have to give it a big thumbs up on that. Newer is not always better. They win in a couple of categories but it ends there. I don't think they will have the same reputation of high mileage as the older trucks. They have too many issues new.
I miss the fuel mileage but in reality it's a second vehicle in this house so not a deciding factor. But I just went completely over my truck top to bottom, and did every maintenance item needed and that could be done, including spark plugs, fluids, and O2 sensors. This truck is SO cheap to maintain! We have two German cars in this house and they can put you in the poor house. This thing is a dream in comparison.
Personally I paid up a little to get under a 100k and I wanted the 5.3 to avoid premium as the mileage is not there. I am very happy with the GMT-900 and the mileage with the 5.3 is not too bad. It does everything else pretty well. I would buy the newest you can the '13's and 14's seem to have a lot less little issues like door actuators. Owning a GMT-900 outright is great in comparison to a truck payment on a 75K vehicle and going to the dealer for a lot of nonsense. ( new truck needing to be ripped apart for a torque converter) (AC condenser in the middle of summer/ national backorder it was so common) Obviously my vote is GMT 900. There is a case to be made for either, but I don't feel the new ones will go the distance, and they are too much for what they are.
best of luck