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.