if it can upscale 1080 to 4k, does that mean I don’t need my 4k collection anymore? It sounds too good to be true.

Anecdotal so obvious grain of salt but:

It really depends on the kind of media. I have found that it does an AMAZING job with 80s anime where you basically had humans drawing vector art in a lot of ways. It doesn't work as well with the batshit insane "textured" 90s stuff and is hit or miss with the cg-enhanced >00s stuff. Same with live action. Something like The Fifth Element (but not actually The Fifth Element since I am pretty sure that was actively part of the training set) will look "a bit off" but a nice drama with "normal" costumes looks solid.

But also: "a bit off" mostly is for s tuff that I "know" what it should look like. And you might get lucky and see a weird artifact for a split second if you pause at just the right time. But it is really not that dissimilar from compression artifacts from a "poor" rip or the crushed bullshit that the streaming services call "4k".

So I still rip my UHDs. But if you are grabbing them off the internet and they've already been compressed to hell or were ripped from a streaming service that is similarly compressed to hell? I wouldn't bother and would probably stick with the 1080p versions.