While I think you are correct in that IP seems more stuck by middleboxes (SCTP is unreliable), I think any protocol whether it be over UDP or TCP can be subject

While I think you are correct in that IP seems more stuck by middleboxes (SCTP is unreliable), I think any protocol whether it be over UDP or TCP can be subject to ossification. QUIC seems to have some useful properties to prevent this, such as the headers being encrypted. I could imagine alternative UDP protocols not doing this and causing more ossification.