A major mistake in IP’s design was to allow middle boxes. The protocol should have had some kind of minimal header auth feature to intentionally break them. It wouldn’t have to be strong crypto, just enough to make middle boxes impractical.
It would have forced IPv6 migration immediately (no NAT) and forced endpoints to be secured with local firewalls and better software instead of middle boxes.
The Internet would be so much simpler, faster, and more capable. Peer to peer would be trivial. Everything would just work. Protocol innovation would be possible.
Of course tech is full of better roads not taken. We are prisoners of network effects and accidents of history freezing ugly hacks into place.