This aspect might be less challenging for a Reddit alternative, under the current circumstances, than a Twitter alternative. As you essentially said, it would be difficult for an individual to move from Twitter to Mastodon because it’s likely the majority of the people you follow won’t also move and without a mass exodus, probably most of the people who follow you won’t move either.
With a Reddit alternative, it (potentially) becomes much easier if groups of moderators for massive subreddits, acting together as they are for this protest, start saying “we’re not doing this any more, we’re going over here instead.”
I think the current circumstances create a good opportunity for moving to a federated alternative but for it to actually happen, it would rely on an alternative already existing with the moderation features they want. Those mod groups would also need to make a coordinated effort to bring up instances for their communities simultaneously to avoid any being inundated with more users than they can afford.