One effect is to encourage competitors to use non-iOS platforms, including webapps on iOS, or android. The consumer downside is if these are not also available for iOS.
However, if Apple quickly copies those features, or buys the company (as it often does), then its customers aren't harmed. And, in fact, Apple's apps are often leading anyway.
I think the place for a real downside is for apps that would be better or more quickly developed on the iOS platform, or would be more attractive to developers there (because more profitable or more users). These apps would appear more slowly for other platforms, if at all. Because native iOS apps are faster than webapps, and because iOS customers seem willing to pay more than Android users, this downside seems realistic - and invisible.
However, if Apple quickly copies those features, or buys the company (as it often does), then its customers aren't harmed. And, in fact, Apple's apps are often leading anyway.
I think the place for a real downside is for apps that would be better or more quickly developed on the iOS platform, or would be more attractive to developers there (because more profitable or more users). These apps would appear more slowly for other platforms, if at all. Because native iOS apps are faster than webapps, and because iOS customers seem willing to pay more than Android users, this downside seems realistic - and invisible.