> could just not ever set the spin bit.

That works until middleboxes start requiring that the spin bit is set. As I commented in the earlier discussion linked above:

"You do know that someone somewhere is going to make a middlebox that just drops a packet unless that bit flipped in the precise sequence that the middlebox developer believed was the correct one, right?

Then someone proposes an enhancement to QUIC which happens to change the sequence of the flips (perhaps some multipath thing, or an enhancement in the way it treats reordered packets), and it breaks... "

Indeed. But it will still expose its initial handshake RTT.