So, what we have is the artwork. The artwork, as a digital artifact, continues to exist as long as there is a copy of it anywhere. A copy that was hosted at a particular URL may cease to exist, though.
"Buying a URL" doesn't make sense because there's no transfer of rights. I already have the URL! There's no meaningful mechanism by which I can take control of the URL, and there are no rights over the URL that anyone can sell to me. The reason people mock the idea of "buying a URL" is because they rightly observe that it doesn't make sense.
The value of the record is not directly related to the URL. The value of the record is that it has the artist's signature on it (or the signature of someone I believe to be the artist, or the artist's designated agent; fraud is definitely possible here!). Notably, the URL does not have the artist's signature on it (how could it? This also does not make sense as an idea). The artwork pointed to by the URL might or might not have the artist's signature on it, but if it does then the signature is easily copyable, because the image itself is easily copyable.
The thing that is not easily copyable is the blockchain record. This is because it contains an encrypted transaction signed by the artist's key, and I can't make a duplicate of this unless I have the artist's private key.
If I'm buying an NFT, I'm doing it because I think the artwork is going to be popular or important in some way. If this is true, there are going to be many copies of it. I don't really care about what happens to any single copy, even if it's the copy that was presented as the reference copy at the time I bought the NFT. If I'm truly concerned that the image might disappear entirely, I will download a copy to my laptop and save some backups. If it turns out that the thing I bought was so unpopular and unimportant that months later nobody anywhere saved a copy of it, then I suppose that sucks for me.