A lot of the longevity of the game for many was mods, and I wonder how this will impact that. I played almost exclusively on retail, but I know there are many people that are passionate about modding it.
I think they could have made everyone happy by just saying if you want TCP/IP, your character cannot use Battle.net. Keep them separate like the old days, locally stored, anything you do to screw with your game and cheat is on you. This would also mean no crossplay, I believe, which is one of their selling points.