The 721 can tale a massive cartridge upgrade, believe it or not - this and the 701 were excellent 'sounding' decks regardless, and the 'lossy' vibe in the headshell and not very tight arm-height adjustment may well help with resonance transmission/absorption/dissipation here (I'd need
@Frank Dernie to maybe confirm please, as Garrard made some surprisingly good 'sounding' auto decks using this technique which is totally opposite to the 'Linn' way...). One chap on VE, filed down the lip either side of the later 721 'TK' carrier and fitted a Rega Apheta to great effect and I've loved a Koetsu Black (original) in my 701 until a coil-wire broke and lost a channel (I keep threatening to have it re-wired as the tip is very low hours indeed but it's going to cost hundreds to do it I suspect and I'd only trust Expert Stylus Company with that kind of work I think, although Goldring do this work in the UK too at a price...). A more solid metal bodied cartridge doesn't seem to make the arm as microphonic in handling as some MM types do. I also have an Ortofon MC30 Super and a well-worn (sadly) Stilton bodied AT OC9 which the 701 arm is perfectly happy with, so a modern Ortofon Quintet or AT OC9 variant should be great for the 721 which has VTA adjustment.
The Pabst-made? 721 motor looks to be an ancestral design that influenced the current Technics one... and don't ignore the Dual's sprung deck plate as, despite the lightweight plinth, the stylus does seem to be very well isolated. I can pound the massy cabinet-top the deck is sitting on and nothing gets through at all. I had to work on my Technics SL1500s to get anywhere near that kind of isolation in the same situation (removing the lid when playing and in my case, bypassing the feet entirely!)
One thing if you want to try a good MC type of pickup in this once top-model Dual, the chassis is 'grounded' to one or both of the signal screens (the supplied cable is fine despite looking cheap). I'd suggest snipping the tie wire that does the 'grounding' on the terminal/muting block and running a separate thin flexible wire from said chassis point to the amp's 'earthing' terminal. Loads on VE in the 'Dual' room and service manuals can be got there too, as well as invaluable help and advice from Klaus (Dualcan) who has a site dedicated to stripping many Duals down for full refurbishment. The wiring diagram is on page 19 of the attached service manual I got from Vinyl Engine (hope they won't mind me sharing here).
There is an after-market metal mount available for the VM540 cartridge, which if like the VM740 (same stylus and body but metal fixing), may calm it down a little

This in itself I'd say is an upgrade worth more than a wholesale deck replacement unless you want a visual change of course
P.S. I'm bothering to type the above as I do believe the top trad Duals of this era are still highly worthy models. cartridge sleds are still available (the new 'printed' versions look good judging by the one I have here), the motor electronics on the mk2 motors seem long lasting and the glass? thrust pad doesn't seem to wear at all if mine is anything to go by. I think the deck could compare with say, an SL1500C and if any real 'change' was to be made, one of the top 1200 versions may need looking at (the new swanky-price 1300 for exampke). The 721 would be
at least a couple of thousand dollars if sold today, I'm sure...