True, and Usenet could be handy. But basically it became a spam forest, and you'd have to wade thru 200 spam emails for one on the topic. Maybe if they would have developed filters for it, it could have gone on further.
No, it didn't. Spam was a big issue for a while but server side spam filters like cleanfeed and distributed systems like nocem became very sophisticated and effective. Unlike email filters, Usenet filters have the advantage in being able to see *all* the destinations. If an article that appeared in more than a handful of groups was quickly squashed. Spam never entirely went away but it well under control long before the decline of Usenet.
There were also efforts like Usenet2 that created a network of trusted servers who would keep spam out. It worked fairly well but interest waned initially because the spam problem was effectively controlled in regular Usenet but even more so as total volume declined and the Usenet2 corner became too thinly populated to be of much use.
Now there is still the problem of idiots posting things in inappropriate places but that's a problem of moderation, something Usenet never did well. (Usenet *did* have moderated groups but it drastically slowed conversation and did not scale well)
I still run a small news server. Spam is only a "problem" is groups where the posting volume has dropped near zero and spam is all that is left. A bigger problem is that I keep losing peers as people give up and shutdown thier servers.