[Montellese]

Ok, let's talk a bit about the console specific controllers. If one has a NES controller I ofc can understand that he wants to play his NES games with it. However, nothing prevents us from assigning multiple controllers to one player. So if you have two 360 controllers and tow NES controllers, bind one of each to player 1 and player 2 and just grab the one you want to use atm. Won't this work just fine?
The only situation where you would have to switch a controller between player profiles is when you only have one of each and a friend comes by.

There are several assumptions hidden in your approach that are not given. For a lot of controllers we don't know that it's a NES/PS/Xbox/... controller. On win32 we don't even know that it's an Xbox 360 controller because you can use the exact same API with a DualShock 3/4 and there's no way in the API to differentiate them. On win32 a lot of controllers are generic bluetooth controllers so we have no idea if they will be used together or separately. We also can't know which controller a user wants to use with which platform so having multiple global player 1 controllers would be a nightmare. What happens if you have two player 1 controllers and they are used at the same time. You have to know that controller X is used as player 1 on the current platform and that controller Y is used as player 2. The obvious choice is to just assign them to the available ports one by one but if a user wants to use different controllers he has to be able to do that mapping specifically because there's no bullet proof way to do it automatically. Either way even in your example there would have to be a way to specify for each controller that it's player 1 or player 2 so you need some kind of configuration tool.

Personally I have a few Xbox 360 controllers, a few controllers from Google for Android TV, the Shield TV controller and a bluetooth NES controller. I've tried playing NES games with the Xbox 360 controller and it's ok-ish but the controller just isn't meant to be used with the D-PAD all the time and it feels odd. So I use the NES controller to play NES games but it's impossible to SNES games with the NES controller because it misses some buttons.

As already mentioned a few times the goal has to be that for a normal user with one or two controllers everything works out of the box. But for users with a lot of different controllers who want to use them explicitly on specific platforms there also needs to be a comfortable way to be able to specify how they want to use their controllers.