Yeah it does seem like antitrust is the wedge employed by 3rd parties who would like Apple to do free R&D for them, most ironically by Spotify which holds are arguable monopoly on music streaming.
I’m glad that antitrust enabled a rich ecosystem of Microsoft Office alternatives and competitors.
Apple certainly doesn't have to do free R&D for third parties. They could, instead, not ship features that work exclusively with other Apple products. Or, hey, there's also the other valid option of "don't be the exclusive gatekeeper of the platform", they're welcome to take that one too.
When Apple goes up against governments over encryption, I'll cheer them on with everyone else. When Apple is engaging in https://en.wikipedia.org/wiki/Tying_(commerce) , I have zero sympathy for them.
You are at least an honest critic here in that you admit you'd rather Apple not build great features for its own customers if that means some hypothetical future company isn't allowed to hypothetically build its own version if Apple made some or all of its proprietary R&D available for free to this hypothetical competitor.
Most people pretend that all that effort is free and trivial to expose as API.
I’m glad that antitrust enabled a rich ecosystem of Microsoft Office alternatives and competitors.