Fair enough. I own lots of controllers, including several legacy ones for retro systems. I care about quality of the controller and also in using the best controller for a given game. I also have systems which do not have USB (e.g. Arcade SuperGun) and I sometimes build my own joysticks with DE15. I know "the best" is subjective, but basically, I like having the option to plug anything anywhere.
For MiSTer we only care about USB, indeed. There are excellent USB controllers out there (e.g. Buffalo) and the 8bitdo receivers allow using Bluetooth ones as well (e.g. PS3/PS4). I'd recommend the 8bitdo wireless approach over anything else, but many people (like me) already have original controllers too. Playing the NES with the Neogeo gamepad with microswitches is pretty cool.
The goal would be to make a lag free adapter which could support as many legacy formats as possible, maybe not out of one box but with a dongle for diffwent connectors. The assumption of course is that said dongle can be made cheaper than a whole USB adapter (or at least comparable). One MCU probably won't have lag, but with two in the chain it might be a problem.