For one, I’m betting that bit rot on the disk will take longer to happen than shenanigans with the streaming rights.
For two, the video quality of most streaming services is god awful. The resolution is there but the bit rate absolutely isn’t. I watched Mr Robot on Prime Video recently and dark scenes (of which there are a lot) looked like a blocky watercolour mess.
Another reason to own media is that companies like netflix can't change the playback experience. There's already annoying "skip intro" buttons, ads on the end credits and age and strobe warnings whenever you touch the pause button.
These annoyances have the tendency to creep in and I would not be surprised if at some point they include unskippable ads and a permanent Netflix logo in one of the corners. Disney, who are the worst offernders here, already displays a Disney+ logo for the first and last 15 seoonds. Once the bluray and DVD market has died, streaming companies may get a lot braver with this kind of stuff. (Remember when you could watch TV without a channel logo and banners advertising other shows?)
Oh and then there's the SJW patrol that gets streaming services to alter or cancel episodes of shows.
So there's really a good argument for buying the stuff you really like and owning a copy.
The box sets of CDs make up for that display-ability and the booklets can be fantastic little history lessons.