You could do it in 3nm (or any modern, in-demand node) on 300mm wafers. You could not do it in an ancient node where there's no commercial demand, on 200mm wafers the industry is slowly but surely superseding. These old nodes exist because the facilities were built 40+ years ago and there's just barely enough demand to keep them rolling.
The Skywater fab ran its first wafers in 1987. You could not build such a facility today for the same reason you couldn't build a large-scale factory for carbureted automobiles or CRT monitors. People still use these things, there's still a market for them, they're still serviced, but you could never find investors to build new production capacity.
