Exactly. But every flow still uses memory, and with TCP you know when the connection has ended and can reuse it immediately. With general UDP (and QUIC because

Exactly. But every flow still uses memory, and with TCP you know when the connection has ended and can reuse it immediately. With general UDP (and QUIC because they intentionally put this in the encrypted part) the only way to do it is with timers. So you have both problems of state sticking around for a lot longer than needed if the timer is too long, as well as the state going away too early and breaking flows that wouldn't be broken otherwise because the timer's too short. I get why they made it hard for middleboxes to modify the connections, but they didn't have to make it hard to observe the state of the connection in order to do that.