Earlier quoted context omitted.
That's assuming you only ever run old Blu-Rays on that player. Blu-Ray has a thing called BD+ on it, which lets content companies run arbitrary native code on your player (among other things). It also lets you update your player's key revocation list for HDCP. So either of these mechanisms could be used to render your old purchased content unplayable.
Do you have any sources for this?
The one thing I'm not 100% certain on is if HDCP revocation list updates go through BD+ or if there's a separate mechanism for those updates. However, BD+ code gets to inspect your player software and modify it, which is sufficient capability to do updates, even if they don't use it for such.
I also don't know if they continue to release updated BD+ code or if they've given up on that.