An open protocol like ActivityPub is a public good. There's no entity that tries to "capture value", so it evolves according to the needs of users. When protocol is developed by a company, incentives are different. A similar thing happens when cryptocurrency is involved, because interests of coin holders are not aligned with interests of users.
Association between Nostr and Bitcoin is weak, but it is nevertheless tainted by it, and in my view that makes it less open. I don't want to work with people hired by Jack Dorsey. Similarly, I don't want to work with deranged bitcoiners. People who are building Fediverse do it because they love it, not because somebody or something pays them, and that makes a big difference.
I'm not sure what do you mean by cultural debt. Decentralized identity has always been a hot topic in Fediverse, and I don't think anyone would fight against good ideas. Technical debt exists but it is small, and it is not critical for those who want to build something different. One can go quite far without breaking federation with popular implementations like Mastodon, and if incompatible features are necessary, bridging is always an option.
There is really no technical reason to create a competing protocol.