I had a Mark IVB and honestly I think the Mark V does everything it does, as well as if not better than the Mark IVB. I feel like I've lost nothing by trading up to the V. However I did spend less time deep diving into the IV than any other Mesa I've ever owned, in fact I only had it for one year almost to the day. I got it at the 2022 Orlando guitar show, and traded for the V at the 2023 Orlando guitar show a year later.
I certainly never felt much love for the control layout of the IV. I can understand how a lot of players would find it very confusing and difficult to master. The V's layout with no shared tone controls between channels is extremely superior. Since I had prior 3 channel Recto experience, plus Mark experience, familiarizing myself with the V was a matter of just minutes of experimentation. I did have to refer to the manual to understand the EQ presets but otherwise, it was self explanatory.
But I really like the way that the footswitch stores in the back of a combo Mark IV. I like it enough that I'm considering engineering a similar storage system for a custom footswitch system for my Mark V. A lower profile footswitch with 1/4 turn fasteners just like the one on the IV should be pretty easy to whip up.
It's starting to look like the signature feature of the later Marks is the combination of the tones of the earlier Marks that they provide.
If the VI was still some distance into the future, I'd expect it to be updated to have a channel modelled after the Mesa Badlander.