Well, developers of modern web apps can offer a qualitatively better experience on those other platforms in some ways than they can with iOS Safari. The offline working features are one clear example. The use of open (=> cheaper and sometimes better-performing) standards for things like audio and video content is another.
Unless you contend that the only reason any web developer ever uses the features available in other browsers but not iOS Safari is to abuse or exploit users, in other words that no-one is (or would, if Apple supported them) make any legitimate use of those features that would improve the user's experience in some way, it is clear that users are materially disadvantaged by the limitations of iOS Safari, unless they have the choice to use another browser that does offer those features instead.
When there's articulable harm.
Microsoft forcing the junk that was IE, and all their other thuggish tactics, is articulable.
The differences between chrome and safari are insignificant enough that you'll bore most people when trying to superficially explain how pretty much the only people negatively effected are advertisers, who have been avoiding their public reckoning over the straight up horrific things they do in closed rooms.
Maybe I'm wrong and there are consumers that harmed by the lack of chrome or Firefox on ios, but that'd be news to me.