I understand it is a rogue-like...
*looks forlornly at the long legacy of Gauntlet, lost to the tides of time*
RIP
1985-2005
Legacy aside, you know how easy it is to kite with the ranger? It'd be stupid easy with twin-stick-shooting. The other classes would require major buffs to compete. I know it doesn't seem like much of a change, but imagine playing a survival horror game with FPS controls and cameras. FPS control schemes are designed to empower the player and allow them to quickly react to their circumstances (with some variations, Tom Clancy shooters are usually less responsive than, say, Halo controls, but that's due to a change in emphasis between strategy and tactics/twitch). On the other hand, survival-horror games usually try to disempower you. Can you imagine playing Resident Evil 2 with Halo's controls and camera? Completely different experience. Even a hybrid game like Dead Space includes several disempowering elements, such as disguising your HUD into your character's body.
I'm not saying a twin-stick-shooter built by this dev wouldn't be fun. I'd probably buy it if he/she/they ever made it. But it would be a completely different game than Hammerwatch.