I can't speak for anyone else but I think I can work around that by moving the site around to different VPS nodes from time to time. I get bored with my silly hobby sites all the time and nuke the VM's then fire them up later which gives them a new IP. I don't know what others might do if anything.
If I had a long running site I could do the same thing by having multiple font-end caching nodes using HAProxy or NGinx that come and go but I acknowledge others may not have the time to do that and most probably would not.
Anyone who wants to track your users can just follow the IP changes as they occur in real time.
That's not quite it. The issue is that there's no other traffic bound to that IP - ECH doesn't buy you any security, because an observer doesn't even need to look at the content of the traffic to know where it's headed.