Sure it is.
When I buy a PDF from Drivethru and download it, it's mine forever.
Until you have a hard drive failure.
Which is a thing that has literally happened to me, and cost me MANY PDFs.
Thankfully, I had some of the older ones backed-up on burnt DVD discs. Unfortunately, a couple of the discs had degraded and couldn't be read.
So the files were lost. And some games and files no longer online were gone forever...
When you buy a book on Beyond or one of those other VTTs it's yours as long as they find it worth their while to exist.
The 4e Character Builder was online from 2010 to 2020 and only went offline when Silverlight, the program it used, ceased to be supported. Which was still six years after the launch of 5e and eight years after 4e ceased to meaningfully be supported.
There's no clear reason to believe DnDBeyond will cease operation any time soon.
Nor is there a firm reason to believe that if the partnership does end, the program won't remain online but cease to be updated.
You used to own a thing forever, now you don't. Who benefits? The consumer?
Owning things forever has never really been a thing.
I can't tell you how many times I bought a copy of
Star Wars or
Army of Darkness. I just moved and threw away several boxes of VHS tapes that I "owned forever".
Or the box of CDs that I still "own" despite not having a CD player apart from the optical drive in my computer I'm debating removing because it barely works and could be replaced by another hard drive. Yeah, I don't "own" the music I buy off iTunes and it could be removed, but at least I can play it still.
Will I care in 2035 when DnDBeyond goes down and I lose access to all the books I paid for on there?
Sure. But not a lot as I won't still be playing 5e. Those books will be in a closet somewhere beside my Pathfinder and Palladium books. If I haven't recycled those or gifted them to Goodwill. I'm losing something, but it's something I don't care that I'm losing as it's not being used.