I don't think it would kill RPGs, but a real DnD(or D20) Online would be nice.
A very loose playing field(probably using editable tokens on a 2d surface, easy to adapt to background maps), with dice roller, manual stat calculations (with could be macro'd for later use), long-term character/monster/campaign information holders, and voice-chat. Probably optional webcam feature for each of the players. To not be market suicide, it wouldn't contain any important information outside SRDs, however it would be possible to store an endless amount of personally typed up notes(which someone could just write up all their books on if they had the time/were sane enough to do it). All games would be played on servers, with nice little options like being able to save and stop mid-game, searching for groups or members, or ect.
Probably a 1-dollar fee for "Player" accounts which would come with the basic access to the game and maybe storage of characters from campaigns. A 10-12 dollar fee for "Dungeon Master" accounts which would include all the major data storage and game-holding stuff. If they really wanted to make money, they could include monthly online additions to Dragon and Dungeon magazine shaped just for this "game", stuff like macro'd stat calculations, (brief) notes, and maps all ready to go after download for an additional cost to the monthly(maybe 2-5 dollars each). While we are at it, do it for all modules and accessories. Maybe even open it up to 3rd party companies (for a cost to do so of course). More than likely less than a few players would be content with just a Player account and will upgrade to a Dungeon Master account. Hopefully the need for crunch/lore from physical books wouldn't be harmed through illegal means thanks to the note feature(which should be reason enough to keep it untouchable through 3rd party mod edits and not allow people to share DM notes within the game). If all went well, Wizards would be rich as hell.
Maybe Play by post would be hurt a lot ;p.
EDIT: Oh my, I should stay sort of on-topic...
So now I am asking myself, what do tabletop RPGs really have to offer me anymore?
If you have fun thinking of stories and/or creating complex but neat systems and doing it with friends, then there will always be a get incentive to play tabletop RPGs.