Join a session of what exactly? If I buy Firewatch <http://www.firewatchgame.com> from Steam, how would you propose I download it and play it on my Nintendo Switch, my PlayStation and my Xbox?
Something like fortnite, once you purchase the battlepass/skin on one platform they are linked to the account and available across all the other platforms the game is available on.
This model essentially poses big challenges with physical media and game preservation. Puts the user at the mercy of the publisher.
IIRC Ubisoft pulled the plug on "The Crew" which is online only at the moment and is scheduled for deprication March of this year!
With regards to physical media, I think the game can skip the licence check on the account itself for the base game if it was launched from a disc/cartridge.
I should, within my own hom, play the game on any device.
I have 5 high-end machines/laptops, a ps5 xbox, ps4, tablets and phones...
If I own a license - I should be able to play from any of these.
If I join a multiplayer game that is OUTSIDE my LAN, then do a session ID check - and if two+ clients connect to game...
Make sense?