The opposite seems true right now, with Apple deciding to improve its proprietary ecosystems (app store, swift, etc.) and letting WebKit stagnate. There's a lot that Gecko and Blink can handle but Safari struggles with, seemingly by design. Apple doesn't want a strong web-based ecosystem because that means their competitors can use it too, vs the iOS/macOS only stuff.

If they wanted to, they could switch Safari to Blink just like everyone else did, but still keep adding features on top of it (privacy, iCloud integration, whatever), the same way Microsoft does.

No user asks for a different layout engine, that's corporations fighting with different values and priorities. The differentiating features for users are layers above that and arguably held back by renderer differentiation.