I don’t know what is going to happen, and as I said, I don’t even care that much to be honest.

Blast radius of what? How does that affect existing Mastodon instances?

It does if this happens gradually, when instances bleed users to Threads because it has “more features”/works better/etc.

I’m optimistic because I think open alternatives are generally better and will win long term.

Good for you, I am not sure what this optimism is grounded on, but I lost it completely. I think the battle is already lost, and open solution can -at best- represent a niche corner of the internet. People are used to things that are addictive and create expectations that are unrealistic for services run with budget at 4 digits top. There is no going back, in my opinion. Either way, this is very much besides the point of my argument, which was that email is exactly an example of how big companies can take over “open” protocols with them being left “open” but effectively having 99% of users on 2/3 providers, and a very high entry barrier which renders the “open” nature of the protocol just a formality.