The investment would be enormous and the payoff would be....what exactly? People who are going to continue to subscribe anyway do just that? Yes it would look prettier for everyone who could run the improved graphics, but that doesn't seem likely to draw in any new customers and only possibly retain a few.

On a slightly different note, it's quite possible to run FFXI in a higher resolution by toying with the registry editor ever so slightly (note that I'm not recommending or condoning this -- it may be against the TOS but I'm not really sure) and of course people make interesting .dat swaps for themselves and other enthusiasts. If SE were to, well, firstly allow .dat swapping in some form or fashion, and secondly release some rudimentary tools for file conversion I'd bet fans could have the entire project done fairly quickly.

It would actually be a rather interesting event if SE were to distribute currently used models/ animations/ textures, allow users to players to edit them (assuming said players had capable software), players then submit them back to SE for approval and they're optionally patched in at a later date. Not that it would ever happen, but it would be cool.