You're right that the two forms of ossification are distinct, but I'm quite certain that both were driving factors here. Rolling out new TCP options is a decade

You're right that the two forms of ossification are distinct, but I'm quite certain that both were driving factors here. Rolling out new TCP options is a decade-long process, and if anything goes wrong with the initial design, once it is shipping you're basically never fixing it. Applications are much easier to update than operating systems, auto-update for the vast majority of users, and for the <1% who don't update the app can always fall back to TCP. (E.g. the rationale for TOU, now-dead Facebook's UDP-based transport explicitly stated userspace-only deployability as requirement. They needed a solution for all their other requirements to be deployed in 2015, not with a consensus design approved in 2020 and in wide deployement in 2025.) The middle-box driven ossification happened by making things harder. The OS upgrade driven ossification happened by reducing/delaying the payoff.