Most of the useful apps has to connect to a server, let's say app devs stop serving requests for iOS, Apple would be pressured to come to the table because the iPhone is useless without apps.

This would take such a critical mass that it's silly to consider. Without some critical mass, the average consumer would see that the rest of their apps work fine, and probably just give the app a 1 star review "it keeps freezing".

Unions are able to organize with similar power inbalance. Collective action is not impossible, just difficult.

Nobody will a join a union just to strike ( a.k.a pull the apps) however making collective to better negotiate with someone with lot of control over your revenue (salary) is not inherently a dumb idea.

I would naively claim that unions have power because they are physically localized, and represent similar people (income, standard of living, laws, etc), and create a very physically localized skill vacuum when they strike, that applies immediate and significant pressure.

App Store developers are sprinkled all over the world, so not too much is consistent between them. Getting enough of these varied people to collectively pause their income, and allow their competitors to flourish, long enough for Apple to even notice/care, is probably much harder. I think it would be even harder to present the applications failure in a way that an average consumer would sympathize with or even understand.

The small indie developers will not influence any Apple decision making anyway.

You only need really apps with say 10-50M downloads to join together, most of them know each other and many of them are made by rival companies who have an interest in keeping Apple in check.

Indie developers could of course be invited to join such action, but apart from PR and other aspects they won't move the needle either way with Apple.

Indeed. If some random app does not work I, as a user, would call that app as crap and not Iphone.

Also majority of those serious apps are part of enterprise's digital platform who are not going to knowingly break their Apps.

These apps will be booted off the store (look at Epic), bye-bye revenue