The code was rife with vulnerabilities, so the boards needed constant patching (which was a non-trivial that sometimes killed the database). If you didn't patch on time, a script inevitably dropped by, exploited the software, dumped all credentials, and nuked the database.
Those old forums were not built with the adversarial nature of the 2010s internet in mind. Boards were dropping like flies a few years there. Most simply never recovered.
Everyone is equal and hierarchy is bad.
It did change a lot but the biggest changes were the ToS/AuP of server/VM providers. What was not even taboo in the early 00's was becoming a problem keeping an account active on clear-web sites. Across the board many providers starting using the vague word "lewd" a word I had never heard of even after running porn sites for a long time.
Many of us moved to .onion despite being incredibly slow at the time. We would keep an unpublished clear-web sub-domain active for the old time users so they had a fast connection. Eventually that was even problematic so many forum operators stopped accepting new users and made their forums private or semi-private. Some still exist and some got married, had kids and real life took too much time and energy to also run such sites.
They complain about hiring a moderation team to respond to abuse reports? How are they doing it now? Do they not respond?
Gone are the says of paying a couple hundred a year for a host and a vBulletin license. The new costs are many thousands plus permanent liability, and no thanks - that's way more than I'm going to do for a group of enthusiasts online.
People's habits changed.
I do agree that things got worse in the last ~16 years or so.