I guess I would say that I really don't need for the Parker 51 to come back. The special edition had so many material problems and the 100 just didn't do it for me. The knock off versions didn't impress me at all. There really are enough original 51's out there to satisify my need.
I take this meaning you don't need any more poor quality knock-offs, branded either by Parker or otherwise, not exactly that you wouldn't take a "proper" Parker 51.
The thing is that 51's are probably the easiest "classic" fountain pen to bring back right: no out-fashioned materials (i.e. celulloid), no difficult to replicate nibs (i.e.: not flexible), no complex internals that would make it too expensive to build (i.e.: two-staged pistons)...
That there's a market for properly designed pens, I think is obvious just showing how well i.e. Pelikans M400 or Montblancs 146 are still selling -even them being "inferior" to the originals (I for one will take a 50's Pelikan 400 or Montblanc 146 over their modern counterparts any day of the week, twice on weekends) so seeing how popular 51s still are, it seems a no brainer to re-introduce them (even more: being them "natural nails by design", they may appeal to a larger audience used to ball pens)... as long as they are faithful to the originals.
Even more, reintroducing them would allow for other variants (just like M400 opens the door to the whole Sovereign line, 400 to 1000, and a lot of variations of each of them, in colors and materials) -after all, as you say, if it were just for the 51s, there's still a ton of good originals you can buy: I would enjoy, i.e. a Super 21 exact replica in lucite and gold nib, for instance.
So... why not?