Xbox 360 marketplace will stop accepting purchases per July 29th
support.xbox.com
support.xbox.com
Without dedicated third-party support, these devices would pretty much be junk.
So from the sound of it you can still buy content. Just not directly on the 360 system
https://www.xbox.com/en-us/games/backward-compatibility?cat=...
As a Rare / Microsoft game, I'm sure Perfect Dark is on the BC list. Phil Spencer has suggested that licensing is the biggest issue for heavily-desired games that are missing, like Jet Set Radio Future.
If there's anyone out there with the magnet or torrent to the 2011 XBL / partner net dump that may or may not have been leaked (not src, just bin), there are an entire new generation of talent out there that wouldn't mind picking up where the now fallen - giants had left off.
"Clean room" implementation folks were able to replace the pinned certs and connect to our own "sex box lies" before the momentum was focused elsewhere (also MS has goons)
For both Xbox 360 and Ps3 I suspect a decent business move would be to have a one time deal. A $100 (or whatever) one time payment that ensures that the core system will remain online for a decade. Doesn't mean any additional purchases but means that match making and friends lists will stay running until 2034. That would have given these systems a 26-27 year life time. Sony is still releasing system updates once a year to keep the Blu-ray keys up to date.
The online portion of the games, explicitly, were segmented.
In fact: the "unspoken" reason why online services cost money (the money is actually a red herring) is so that the online services are materially separate.