Well OK....but that's not "nothing spins"
On my MX-80, while the tension arms do spin, the guides on either side of the head block...don't spin.
I don't see that as some indication of overall deck quality though.
The whole tape tension/movement is set up per machine based on all that is part of the deck's path. IOW, adding rolling tensions arms to a deck that was designed with non-rolling tension arms...will not really improve anything, IMO.
Heck, you might throw the whole thing out of whack!
Plus....heads are always stationary on every tape deck...so you still have tape drag there, more than at any other place in the path.
Don't get me wrong...I said the Otari floor models are certainly more robust, especially for daily use...but I don't think the later models of the 5050 line are all that much "less" in overall audio quality. This BIII, their last production 5050, is pretty pro-end in quality.
The nice thing about those 1/2" 4-track floor models, is that you CAN get them changed over to a 1/2" 2-track .....now THAT would make them a killer mixdown machine!

There was a guy selling one of those conversion OTARI decks around my neck of the woods not too long ago...and it went for an OK price, I think like around $1k...which is less than the cost of converting it from 4 track.
I took a close look, but had other things on my "to buy" list....but I've always lusted after a 1/2" mixdown deck (especially since I have plenty of NOS tape from BASF/EMTEC.