On iPhones, afaik, you can only get Safari, or other browsers that are just a thin wrapper around Safari.
That is a huge difference and I hope they know that.
This is a similar situation on Android.
The fact that people think that tells a lot about the quality of Safari: "if people could use the real Chrome, they would all ditch Safari right away". That may lead you to wonder why Safari is so bad, considering the infinite funds Apple has? Perhaps is it because Apple has been able to underinvest in it for the last ten years because of the lack of competition?
The harsh truth is that you need a properly executed competitor to both iOS and Android. Unfortunately from the open-source and free-software world, there are just too many failures or potentials never being 'realised'.
We saw Firefox OS being cheered all over the place here as an alternative to the mobile device duopoly and it turned out to be a failure due to little to no usage. The same thing with the promising Ubuntu OS and now again we have low usage with the plethora of Linux-based phones. So it seems that the market just does not care, and Mozilla and others are still unable to bring in something viable to compete against the big two.
It is not 'early days' anymore and it may even be almost too late, unless massive amounts of funding gets poured into an alternative. The same thing is happening with smartwatches with Apple again having a 5+ year lead. We've all waited for Mozilla, Servo and others to become useful and they have all underperformed.
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.
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.