I started racing mountain bikes in 1990 and by the mid '90s even 'small' local races had 1000+ entries while regional races could easily triple that, and NORBA Nats were crazy. When I was finally able to kick the habit five years ago, those same local races were lucky to crack 100. Why? Racing is hard! Yes, its fun, but many people can't overcome the hard part. People (and sponsor $$) look for other fun avenues. Obviously we can't use 2020 as an indicator for anything, but gravel racing will likely meet the same fate as the other dirt (mtb and 'cross) booms did in the USA.
'Cross specific: the USA 'cross boom in all but a few pockets was fuel by short races in easily accessible places (parks etc.), but once your toes get frozen, or you miss your dismount a few times...its not fun anymore either. Its much easier, and more fun for the masses who want to ride to roll down a nice road or path chatting with their friends (nothing wrong with that).
Cycling in general is a niche sport, and I'm OK with that. If 'cross is a Euro sideshow, so what, its a good show.