More information just came out from Epic. The salient points:
1) 350 million registered users.
2) Signifiant number are on iOS, almost a third (probably explains why they are going after Apple first - this is possibly their most lucrative market).
3) Almost ⅔ of iOS users access Fortnite only via their smartphone, so Epic risks losing them forever as a (paying) customer. This would represent a 20% dip in total user numbers.
4) Apple is reportedly preparing to blacklist them for at least a year.
You have made your bed, Epic, now sleep in it.
Interesting... that's the complete opposite of a study of battle-royale games done just a year ago.
It was a study of Fortnite, PUBG, and Apex Legends:

Newzoo’s Battle Royale Sentiment Study: Understanding the Phenomena and What Draws Players to Particular Franchises | Newzoo
Get insights into battle royale franchise loyalty, what made people start or stop playing, and more! Based on research in China, the U.S. & Germany.

"In fact, more than two-fifths of all U.S. gamers are an active battle royale player (played one of the three franchises above in the past month), with 71% playing mainly on console, 17% on PC, and 12% on mobile"
If Epic's data is true... then being banned from Apple's App Store is a devastating blow.
But like you said... they did it to themselves!