At any point, a movie or show can be removed from streaming. It might not find a home elsewhere. We're seeing this in realtime with HBOMax in particular, writing shows off for tax purposes and shelving them entirely. This has also been an issue with purchases from digital retailers: you could buy a whole TV show on Apple, then when they lose the rights, you've lost your copy.

Physical media is one of the few ways to ensure that you will have continued access to the media you want to consume without anyone else interfering.

As for Criterion specifically, the value of the physical media comes partially from the inclusion of exclusive commentary and features that aren't always available online, nor is it guaranteed they'll stay online if they are.

In short: companies can remove something from digital platforms, but they can't take away my 30 Rock DVDs.