[matttbe]

Yes, that's correct. In the Linux kernel, it would not be possible to switch to MPTCP by default.

But apps can use it by default. For the server case, it really makes sense: https://www.mptcp.dev/faq.html#why--when-should-mptcp-be-ena...

GNU/Linux distributions could even switch MPTCP on by default (via eBPF).