Answering that for the coins I'm buying currently (
https://appcoins.io pre-sale):
1) Android app store is huge. More than 80% uses Android and when they buy things inside the games, as to go through the app store
2) Paying every investment in advertising (developers) and every purchase in games (users) with the token is central. The token is used for everything.
3) The protocol will be implemented by different app stores, so I think it would depend on the app store. The first app store to adopt it is Aptoide with a team of 85 people and 3 offices, able to reach a user base of 200 million unique users)
4) $18M hard cap for 40% is not cheap but it's align with the user base and potential of the token
Disclaimer: I'm investing in AppCoins so I might be biased.