Usenet was designed in a time when the Internet was only used by a tiny number of people, all of whom depended on prestigious institutions for their access to it and thus had strong motivations to not to abuse that access (since doing so would be something like career suicide). Many core assumptions in its design rest fundamentally on this always being true, which today it very much is not.

Spam is a symptom of our social degradation, and it's not a thing of usenet nor internet nor IT, it's about our society that reforms schools to form Ford-model workers at any level because they are easier to manage, that promote ignorance just to sell more stuff etc. Something that ANY communication free, user-centric, platform, usenet included, can contribute to reduce. When we are citizens we tend to be honest (on average), when we are consumers we tend to be raw and the right answer we all know that's not put "more cops" like in the prohibitionist era in the US or Victorian era in UK but empower people. We know even from today's data, there are more unwanted teenage pregnancies in US state with "chastity tendency" than in countries like French or Sweden with a strong sexual education at school, there are more violence in "strong police" (sorry I do not know how to name it in English) like Russia, USA, UK, Arab world than in "soft police" countries like Swiss.

Someone think that's being child it's easy, pleasant, so a nice selling point, also children are easy to manage so good for managers/politician etc, but there is a problem, children needs parents to live, they can't stand on their foots in the wild.

On a more limited point of view usenet work well enough for any tech savvy user and commerce see no opportunity in developing it, preferring instead push less free and more profitable web tech solutions, so on one side peoples get advertisements of "new colorful, 'featureful' platforms" and on the other nntp client like desktop MUA does not improve in any way.

Take as an example actual MUA for the masses like Thunderbird, they look '90-style, with all limitation of ancient clients, compare to webmail's evolution... Of course we have notmuch, mutt&forks, pine&forks, but they are not for the masses, so vast majority goes for actual pre-made webmail food...

I'm seeing the same problem with blogs. A lot of them use to have decent comments sections but the owners have given up trying to filter out the trolls, spammers, and morons and have disabled comments. There are things that can be done other than just give up.

Abuse aside, there is signal vs noise. The Eternal September will overwhelm any system that cannot self-regulate its own content.