This misses the point in my opinion. The point of a protocol is to establish a set of rules that need to be followed, that's it. After this, it can be managed in many ways, it can be open or it can be closed, etc. The fact that ActivityPub is "engineered from the ground up to support multiple apps with different functionality" it's because ActivityPub is an open protocol. Every protocol is designed to support whoever implements it. This doesn't have any inherent "the protocol (changes) will suit everyone" or "everyone will be able to keep up with it" property, though. If changes to a protocol happen very fast, apps that are compatible today - and can be compatible tomorrow too - still need to implemented those changes, or at some point they will not be compliant anymore. This is not because the protocol loses the property of supporting multiple apps, but because a protocol still needs to be implemented, which is responsibility of the consumers, which requires time.

So my point was to challenge OC perspective that since ActivityPub is designed to support multiple apps, then there is no risk that it gets messed up and breaks compatibility with those apps (because it's generic) due to - in this case -Threads influence. This is just nonsense, in my opinion.