Where are you guys getting the idea that turn-based games don't sell, exactly?
I doubt there ever will be a (mainstream) 4th edition CRPG. Licensing costs for D&D are very expensive (I read that on some Bioware developer's blog) and turn-based games don't sell very well. I think it would be near impossible to make a 4th edition CRPG that isn't turn-based.
Final Fantasy? Dragon Quest/Shin Megami Tensei? Dozens of successful web-based/social network games? Pokemon, which is about as mainstream as you can get?
Lots of the top-selling RPGs are turn-based.
Let's be absolutely clear. A real-time D&D 4 game would have to be designed from the ground up to be at all decent, and even with that, it will lose all of the depth and fine-tuning of the system.
Look, 2nd edition was simplistic enough that it worked well with real-time (see the Infinity Engine games). But look at the real-time 3rd edition games. Without exception, their combat was mediocre at best. Movement and positioning more or less disappeared as a tactical consideration, strategic subtleties and player feedback were lost under the frantic pace of real-time, and at best you could sort of use some vague tactics if you paused a lot and slowed the game down to a crawl. NWN2 was incredibly faithful to D&D 3.5, but its combat sucked because it missed the critical element of turn-based gameplay.
If a D&D4 RPG is turn-based and faithful to the rules, it'll have the best RPG combat of all time, and properly-marketed, it'll draw a sizeable audience. If it's real-time, it'll join the mass of failed RPGs with lukewarm critical reception and sales. If it's an MMORPG, it'll be a flop. It's really that simple.