If you're so convinced that implementing BC in the specific way you want is so easy, you're more than welcome to apply:
https://careers.microsoft.com/
The reason you can't buy 360 games from the Xbox One is simple - the Store for Xbox One was completely rebuilt from the ground-up since the 360 is based on decade-old architecture. The catalogs are currently completely incompatible. Transferring licenses from Xbox 360 to Xbox One is a series of hacks that are luckily working pretty well, but were hacks explicitly enabled to support backwards compatibility.
But you said it's simple, so there must be something Microsoft is missing. I'm sure the team would love to hear your input!
Other than that, I'm confused about how "the execution has been a fucking mess since day one". "Day one" in this case was only a month ago - the day NXOE launched. It launched in exactly the state Microsoft promised - 100+ games at first, new games being added every month.
You can easily purchase a game from xbox.com or if you already have it, you just download and play. If you have a disc, you just put the disc in, download, and play.
It's all worked
exactly as Microsoft promised 6 months ago when it was first announced. So execution, quite literally, could not have been better. The only thing I can think of is a few performance hitches here and there.