OK, but Diablo 2 access to Battle.Net was free, no monthly fees. That's one of the main reasons why the game was so popular.
Also, if you give the software away for free and rely on monthly fees you will lose a large part of the market. Some people (including myself) prefer to play single-player or to host a game and play with a friend or two. Besides, I'm willing to pay $50 for a game and use it for 5 years, but I'm not willing to pay $10 a month for 5 years, i.e. $600.
What they could do, I guess, is sell a deluxe version of the game that gives you 3 or 5 online accounts. That would mean a single game instance to install and patch even if you have multiple users on the same PC. They could enforce that those accounts cannot be active at the same time, of course.