I don't see a reality where companies like Facebook or Snapchat start distributing their applications outside the App Store. Maybe we see something where e.g. Facebook tries to spin up their own App Store, for them and anyone else who wants to join, but I would bet every dollar I have that this would simply be a failed venture. Because ultimately most people think, intentionally or not, like you do: Apps come from the App Store. Leaving would be suicide.

The company to watch is Epic Games. If they have the opportunity to bring a new gaming-focused App Store to iOS, to get Fortnite and other games back on the platform: I think they'll take it, and its possible it will do rather well. Giving game devs a distribution channel with a far lower cut of revenue is tempting, and may open the door to higher quality games on mobile. That could pivot to companies like Facebook distributing Instagram through something like the EGS for iOS; but again, I wouldn't bet on it.

Three industries will be massively benefited by this change: Gaming, Gambling, and Porn.