Nvidia's core strategy right now is to reuse the gaming dies for workstation cards, or maybe its the other way around. Every 'pro' card they make except for the $15k HBM-clad A100 flagship shares a die with a gaming GPU. The 4090 has 24gb but right now you can buy the exact same chip with 48gb as the $5k A6000.gaming flagship
The 5090 will be capped at 32gb for sure. Gaming might not need the crazy bandwidth of 512bit GDDR7 but it makes the card sound extremely strong from a marketing perspective as being at ram parity with AMD right now is no good. 512 bit bus means leapfrogging them not only in capacity but raw bandwidth as well, its a no-lose situation. Then on the other side, moving up to 64gb seems mandatory for next gen AI cards and very conveniently fills the $10k gap in Nvidia's pro lineup, no extra dies or MCM packages required.
Though I do agree its likely going to be on some flavor of N3. Technically they have more room to squeeze out of N4 but I honestly don't expect 5000 series until 2H24. Surely by then the node will be in full production will all the kinks ironed out.