80 and 443 are only used by convention for HTTP/HTTPS - you can use whatever the hell you like. There's also the option to not use HTTP(S) or DNS at all to obtain addresses, the list of ways you can avoid traditional methods is endless. Finally, you can just serve your DNS on same IP as back end of the app - block the IP and the app dies completely, meaning a simple IP block will not work etc etc. It's super easy to write some code that combines tens of methods, ensuring you get that DNS record no matter how hard the user tried to stop you.

FWIW people (including me) already do this, but its a blunt tool and not all that effective in many cases: https://gist.github.com/ckuethe/f71185f604be9cde370e702aa179...