TCP fix requires a lot of coordination. First you need microsecond time stamps. Then you need an RFC to reduce RTOmin below 200ms. Then you need ATO discovery and negotiation. A lot of moving parts and you end up with a protocol that’s still worse than QUIC. Also note that Linux maintainers have refused to accept patches for all of these things and QUIC is to some extent a social workaround for their intransigence.