macOS-specific browser share numbers are hard to come by, but on our own moderately sized website we see about 60% of macOS users choosing Safari. Even if iOS allows other browser engines, presumably a similarly large number of iOS users would also choose Safari, either because it's the default or because they like it. So regardless of what happens with iOS regulation, it seems likely there will always be significant Safari usage on the web.
So then the only way we end up with a Chromium-only web is firstly assuming a worst-case scenario for Firefox essentially falling out of use, but its usage on desktop appears to have stabilised around 7-8%. Secondly Apple would then need to decide to ditch WebKit for Chromium. I can't imagine they'll ever do that. Apple have a strong desire to do things their own way and keep full control over how their browser works. So I think we're a long way from a Chromium-only web.
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.