The bots pay monthly for a blue checkmark so that they're promoted to the top of the replies. A small cost does not appear to have been a meaningful disincentive, especially when scammers can easily make that money back and political operatives can easily bankroll that.
$5 is enough for a human to glance at the feed and mute it if it seems bad. The trouble with $0 is you can create a million bots and no one has time to look at them.
I wouldn't mind something like that. When I joined Whatsapp it cost $1 for life and I thought that was a good system. Sadly it's gone, replaced by free but ad funded.
