(Admittedly, if I recall correctly, Dejanews had already gone broke trying to maintain that archive before Google bought it, so arguably Google didn't kill it, they just bought the dying corpse and kept it animated in a zombie-like state for a decade or so past it's natural death...)
This is software, not ancient manuscripts written by scribes on now crumbling vellum - there is no excuse for there to be one canonical copy of anything. Every pornographic movie ever made has multiple redundant backups on decentralized peer to peer networks and darknets.
I understand the importance of maintaining references, but realistically, expecting URLS to be permanent is shortsighted at best, unless you own that domain and the server it's on and expect to have the money to keep the rights to it in perpetuity. You can't expect a third party host to be willing to keep the servers on forever.
But as far as the historical record and the data itself - Google's given warning, people can move their data or lose it. Fork and move on.
Google's an ad company, they don't have an obligation to be the arbiters of human history, whatever their slogans might be regarding 'organizing the world's information'. They care about the information that makes them money.
If it's their archives, yes.
And if those are the only archives of Usenet in existence, then it's not Google's fault no one else cared to back it up.