so instance mods will be incentivized to comply with Facebook’s demands to attract new users and maintain their current one
This is where your argument falls apart. Why? There is no incentive for instance mods to want to grow their instances exponentially.
If Facebook's ActivityPub grows to be incompatible with the existing implementation, who cares? So what if you run a Mastodon instance and aren't getting millions of new users a day?
This is much ado about nothing. While there is a shared platform, enjoy the ride, and if they don't want to play by your rules anymore, there's no harm to anyone in saying goodbye and staying your course.
Generally more positive than Twitter and easier to use than most Mastodon clients.