The utzoo usenet archives surely survived to get to us because of the care taken to archive them all along the way. We even have details of archival work recorded inside the archives, like today's post about the A-news archives being moved to two redundant archive tapes.
That post counts "nearly 11000 articles". Thirty years later, so do I:
utzoo-usenet/news001f1>find -type f|wc -l 10918
Unfortunatly, the post does not give the total number of bytes of A-news that were archived, although it does mention they had used 28000 blocks of disk space. So it's hard to tell if we have a complete set of A-news.
We know the B-news set is incomplete; at some point they ran out of tapes and overwrote some old B-news archives with newer data, although some of the old data was recovered from the end of the tapes.