UDP is as clean as you can get it. It is more or less free of any overhead. And networks know UDP already. A new IP protocol is far more likely to be rejected in the network.

This, exactly; it's the actual reason UDP exists. It's a design smell for anything to ask for a new IP protocol number.

A protocol separate to UDP and TCP altogether would suffer from middlebox interference problems.

Aaaa, you mean the problem of -smart- stupid pipes. These do and will exist all the time and this is an opportunity for them to realise how detrimental they do is to Internet.

They're inextricably woven into the fabric of the internet, and unfortunately can't be wished away.