Sorry a bit long winded...
I had a 2006 Yukon XL SLT before I bought my 2018 Yukon Denali and I definitely understand what the OP is trying to say. The seats in the GMT800 were much wider and more plush than the 2018, I could put that seat into cozier more comfortable (to me) positions than the new one. It just seemed like everything on that vehicle was in the perfect spot and was it pretty technologically advanced for it's day. The 2006 had the Premium Smooth ride with the Nivomat rear shocks, it sat on 17" tires and it rode very nice and smooth. I could hitch my boat to the rear and tow without the Yukon even feeling it... however the 5.3L engine and the transmission setup though adequate had some issues towing because it definitely like to hunt more than fish. I loved that vehicle, if it were not for being totaled in an accident I would still have it.
That being said I definitely like the 2018 Denali, the MRC although stiffer at times also rides smooth as silk on most surfaces, it can hold turns like a car with very little lean and the 6.2L 10 speed combo tows my boat with barely any effort. I do structurally feel the boat more than the GMT800 but at 70 mph I can set cruise in Tow/Haul mode and there is no shifting at all. The fuel mileage on the 2018 towing is as good as my all around average on the 06 so that is definitely a huge advantage.
Keeping in mind that the 2006 was an XL so it was longer, heavier and had 17" wheels with a higher profile sidewall on the tires definitely helped the ride. Our company vehicle is a 2018 Suburban with 18" wheels and it also rides a little softer than the Denali, again a longer wheel base but the overall driveability of both is not that of the Denali performance wise.
I accept the fact that I have a large safe SUV that will tow 8400Lbs comfortably, I can adjust to the ride. Mine has 20" wheels and I replaced the Continental OEMs with Michelin tires which made a huge ride difference.
I miss my 2006 almost daily but I am definitely satisfied with the 2018.