As for 100% compatibility, most people will regard unexpected results as bugs or hindrances then switch to a compatible browser. You may be willing to deal with those incompatibilities. I may be willing to deal with those incompatibilities. Both of us are in the minority. Even technophiles who normally like playing with technology seem to stick to browsers based upon Chrome's engines (at least when given a choice) simply because too much of their life depends upon the browser.
What I predict would happen is that Apple starts investing heavily in WebKit to compete and Firefox would have a compelling offering there like it does on Android.
Most people would stick with Safari because is is the default and they wouldn't even mostly be aware that it now may be worth switching due to say uBlock Origin. Safari would continue to serve them just fine.
Even then, from a technical perspective, I’m not so certain that there’s ANY benefit to the user; I’d rather see some headway made on wasm3 support than to let Brussels or London dictate architectural considerations.
Think it through.