The Master Chief Collection is already magically held together by duct tape and popsicle sticks so adding Halo 5 on top of it would likely be disastrous.
As for a standalone release, not only would it cannibalize Infinite's already dwindling player base, they'd have to figure out how to handle the microtransaction economy, something that was key to Halo 5's multiplayer suite. If they were to strip it out entirely, they would need to redesign whole portions of the game. If they left it in, the optics of a port of a nine year old game launching with a microtransaction store would be pretty nasty, especially when it's running concurrently with Infinite's money-grubbing live service.
Basically, too much red tape and headaches involved at this point. 343 certainly doesn't have the manpower to do it in-house either.
How bad IS the MCC? I hear stuff about graphical issues and stuff everytime and I have yet to really understand how actually bad it is for real.