It's already been trivial. There's a few sites showing you a special message or denying access if you block ads. uBlock doesn't really help here that much and if they tried, the issue is very asymmetric - it's much easier to update the site than to patch it again.

So far, most of these sites that try to exclude adblocker users don't detect uBO on Firefox.

Most anti-adblock also fails on Brave. Those that don't can generally be sidestepped with literally two clicks to disable scripts. The remaining few require substantially more tweaking (generally disabling a specific script while allowing others to run) which is outside the domain of most people, but still viable.

Marketshare. People don't care that much.