MPTCP is a drop-in replacement -- but its original designers were paranoid about e.g. packet-sniffing middleboxes, so they made it opt-in.

Suppose for example there's a packet-sniffing security appliance along the path, and it only sees packets on one path of the MPTCP flow, and it says "This stream of packets doesn't look like a legitimate TCP flow, so it's probably an attack, so I'll block it."