I definitely see this as the biggest downside.

However, as someone who doesn't deal with these big companies to begin with, it wouldn't affect me at all if they move out. It would just fast-track me to recommending alternatives to everyone I know.

I already migrated my whole family into iCloud for photos and own Nextcloud server for Google Drive alternative. It's been great. Obviously not the average user, but I could see people sticking with Apple's services over Google's.

Might get a huge base of older adults off of Facebook for good if they decide to leave the App Store.