connecting to default server, which is what 99.9% of users are going to do.

not quite. the default server feature is only a year old. while deltachat itself goes back to at least 2017, so the majority of users will not be on that default server now, and it would be possible to offer a randomized selection to prevent one default server from dominating.

Majority of new users will be. It's still a niche product.

Also, I'm unsure if it's smart the client just picks a server for you at random. AFAIK this uses email as back-end so it's not like you can just swap your email address host like you can swap telco while keeping your phone number. One option would be to have the user first whitelist the email providers they'd trust, but most users usually prefer trusting the app vendor as they're trusting it with the client anyway.