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.
Safari and iOS mobile are the main outliers. Firefox is a little behind too, but few people use that anymore.
Chromium/Blink development is what actively drives the web forward, with everyone else playing catchup. Apple just doesn't care about that (or is perhaps purposely trying to slow it down) in favor of their own priorities and platforms.