Somebody else did have a usenet archive - Google bought them, mashed up a bunch of other nn-usenet Google stuff into it, then let it bitrot until it was effectively useless, and then removed the only search features that worked...
(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...)
And in the intervening years, no one running a news server thought to back theirs up, no one crawled Google and made torrents?
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.
Why would they run their own Usenet archive? Google had the Usenet archiving business sewn up, and by the time they started showing signs of being untrustworthy it was too late and much of the 80s and early 90s stuff only exists in their archives. (Oh, and as a side note apparently a lot of the interesting porn from that era has been lost to history too.)
Trusting any one service to be the bearer of internet history carries the same risk, that the service can go down at any time, and take its data with it. If this information is to be preserved it has to be hosted somewhere, preferably multiple places. If it's to be accessible online, then someone has to pay for the servers and the power and the maintenance.
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.
(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...)