Well, OK, an axe-swinging barbarian should have a harder time spellcasting than an erudite mage. But that difficulty shouldn't be caused by a crappy interface. It wouldn't be much fun by any means I can think of. Damage to metal blades and armor getting clanked against each other is also realistic, but they did away with that, I imagine by popular demand. (Who knows, though.) It's a fantasy world, and magic is fantasy through and through. They can set up the rules for that to be as enjoyable and as suited to the available control devices as possible.
The gaming plateau has at least 2 causes that I can think of. Or maybe they're one and the same, because one is at least partly responsible for the other? I'd say diminishing returns are the biggest issue. Technology itself is no longer an obvious barrier to audiovisual greatness. They can improve resolution, frame rates, rendering fidelity and memory for bigger environments and more things at once, but there are no fundamental barriers left to knock down, as long as the display device is a flat screen and the audio device is an array of speakers. Any premise can be realized beautifully given the right talent and budget, with current technology. No more bleeps and bloops replaced by orchestral music, no more 2D-to-3D eye openers, or flat shading to detailed texturing. I.e., no more intense wow factors to get people excited about newer devices for their own sake. The software drives the hardware, and software is still selling the current hardware very well.
The other issue is the long current console cycle coupled with the decline of PC gaming, at least when it comes to traditional big games. Without new hardware, the look of games isn't going to change very much once the system capabilities are exploited fully. And if games are being tailored for where the money is first, consoles, the ports to the newest PCs aren't going to look much better. Why such a long cycle this time? See above.
/ot