Oh god, if they increase the level cap to 80 like they did with WoW: BC, I'm gonna start crying and then call it quits. Unless they invent some kind of magical item that lets you upgrade your gear with a x% increase of each stat to match up with lvl 80..
I haven't played WoW since Wrath and frankly, I don't remember much, but don't they have Tier gear sets with each level cap increase? That sounds like a decent way of working it out, except that it isn't how D3 works at all.. and implementing it sounds like it would be a huge mess.
WoW (and basically every other MMO in the same mold) functions by making better gear every few months. Items have their own level (literally called Item Levels) and each time there is a new raid or PVP season or whatever they introduce higher ilvl gear that has better stats than what the previous gear has. D3 doesn't function that way at all, when new gear is added to the game it still has the same stat ranges as the gear you could get before, it's not like the base stats of the item are going to roll higher like they would in a MMO.
Which is basically my point, since the stats are always the same in D3 there really isn't a need to just arbitrarily reset them by increasing the level cap. The only way I could see resetting our gear as acceptable would be if they made sweeping changes to the stat system again to the point where the stats on our gear were going to be worthless anyway.