Agree. That's exactly the point. I don't want to disable FileVault to be able to fix the Login screen wallpaper, and there are reasons why I often power off the laptop as opposed to just put it to sleep or lock the screen, so I need to see the ugly thing every time I start up the machine.
By the way, I actually don't have any issues with the default Monterey purple thing as most of people in the thread, but in fact, since I turned on FileVault I have the "Chroma Red" Light (Still) one, which I find really awful. If it was a thing with the Apple logo or a plain dark background I probably wouldn't mind at all.
I used to be one of those people thinking a stupid background is inconsequential, but after half a year of this I'm bothered by it more and more. Not sure if not being able to change it makes it even worse, but I can't believe Apple hasn't fixed it yet after all this time and so many complaints. Is it really that hard? I wouldn't mind having to go through a manual workaround like disabling FileVault, changing the wall paper and re-enabling it, but it seems that wouldn't work the way FileVault and the new separate system/data/recovery volumes work, as per another thread I read regarding the topic.
Anyway, I guess if enough of us keep posting about it it will get eventually fixed, or maybe we just need to wait for the next O.S. to come out around September, which I'm sure is going to have its own annoying bugs... ... just maybe not this one: the infamous wallpaper-gate :D