Grohl has the enthusiasm but not the chops and he’s got his own career so I dont think that was ever actually a possibility. But Portnoy would have totally done it. I dont think he’d have made them go metal, he’d probably have tried to nail neil’s parts perfectly, but then add his extra showy-flare here and there.
The problem with Portnoy is that his personality is too big for the band, he’s such a chatterbox he’d be doing endless interviews praising neil and talking all the ways he’d honour him and so on, but it would also feed his ego at the same time which would be annoying. Portnoy does well in his various prog supergroups where he is by far the most dominant musician.
Portnoy and Rush both love letting Hugh Syme shit out AI slop for their artwork though, so they do have that in common.