You don't need your followers to change app, but you will find that if you tell your followers that you are moving, they will follow you anyway.

People aren't married to their clients. Users aren't on instagram because it is run by meta, they are on it because they can find the content they want on it and their favourite ifluencers are building for it, so even if the content is elsewhere, the platform is likely going to give the best experience.

Same will happen on nostr, but for those who are torn between their fandoms, or who are somehow married to their clients, they get to stay put. Freedom.

I think this also gives clients the freedom to focus on what they want too: features and community or relationship building tools. People will use one app to poke and share with their friends, and the other to support and fan out on their favourite creators. Yet the same content will be on both apps, maybe the apps will even interop so they can each focus on their specialities while operating on the same data.