And this was kind of what I was getting at. Emulation existed in many forms, but usually not perfect with every game, hence innately a little squirrely. This wouldn't be considered acceptable (or, given the state of most console ports, maybe it would be?) nowadays. I'd say the newest batman game is an example of that. The hardware requirements to run it smoothly at release were horrendous compared to what the console required for the same software. If these xbox offerings are the same kind of pathetic afterthought that most console dev groups give toward their PC ports, it'll require 16GB of RAM and a gtx1070 to run Gears of War 2.
I get it, but for the sake of clarity, I'm going to make a list of emulation goals:
1. 100% perfectly accurate emulation of all games (original textures, scaling, frame rates)
2. All games playable in some way, but scaling, textures, and frame rates may be better or worse
3. Not all games are playable or even available, but those that are play like #1 or #2
I'm pretty sure that there are still many games in the Sony, Nintendo, and Microsoft libraries that don't play at all through their emulation efforts, so we need to be realistic with expectations (as we speculate upon a single tweet LOL). Let's say that the 360 library becomes available through the Windows Store. I think we can assume that there will be minimum CPU and GPU requirements, probably some sort of benchmark utility to download and test your PC.
Likewise, will all publishers allow their games to be emulated on another platform? Why would EA allow you to play emulated Battlefield games when they can sell you a copy through Origin? There are going to be both technical and licensing issues to work around. If this does happen in any way, it's probably only going to be first-party MS games at first and it's not going to let you just use your existing 360 discs or transfer digital titles, you'll most likely have to buy them all over again through the Windows Store/Xbox app.
As you point out, not all games are created equal, so the requirements for each will probably vary wildly, but all they really need to do is hit a performance target based upon the platform. If a 360 game that played like crap in 2006 plays like crap on the PC in 2017, then you're getting what you paid for.
I will point out something really great about the current N64 emulation scene. Using Dolphin, you can emulate the Virtual Console, which then emulates NES, SNES, and N64. I haven't tried it yet, but it may end up being the most accurate N64 emulator available?