- some games change with updates (try to play destiny 2 red war story line with your physical disc that you can still buy for some reason despite game being free)
- Nintendo can block specific cartridges (only thing that step Xbox and PS from doing that now is that it's not implemented on their end)
- some games have separate online pass and/or DLC codes that can only activated once
- on PC CDs used to come with a cd-key you had to activate (still do?)
- See Xbox One 2013 DRM plan
Only way to "own" a game is to have a pirated version of a game regardless of a platform.
You might own the disc, but what about the optical drive that has to read that disc? That is already happening with the 5th (PS1/SS/N64) and 6th generation (DC/PS2/GC/XB) consoles, the optical drives are dying and there are no proper replacements. Congratulations, you own the disc but you can't read it anymore.
