I agree if the point is that this might increase the reach of such technologies. But if you are thinking about traffic coming from sources different than your browser then why would they be unable to open a connection on hardcoded IPs?

Also VPN sniffing can be arbitrarily hard, for example if I remember correctly tools like https://www.softether.org/ are designed to work around the Chinese internet firewall.

From my point of view DoH add nothing outside the browser.