Mozilla wins. Apple has to allow other engines on iOS. Most users switch to Chrome. Most developers stop supporting anything else. Mozilla loses.
(I realise this is a controversial opinion but I don’t lament the slower feature release cadence of Safari because IMHO the web is already far too complex and feature rich for its own good.)
It's just a very strange argument for justifying Apple abusing its users for search engine revenue.
Web developers don't just use 1 or 2 mobile devices they own. They develop with desktops where they could use Firefox. They test with desktops or emulated Android devices where they could use Firefox. They find ways to test Safari even if they don't have anything that can run it. They used to find ways to test IE even if they didn't use Windows.
Bringing Gecko to a new platform is much harder than wrapping WKWebView.
They are all behaving anti-competitively in different ways, and they should all be looked at!
Now Google has much less influence over web standards. Instead of shaping the web to fit the needs of its own products, Google would need to work with whatever the browsers decide to allow.
Independent Chrome, for its part, wouldn't be able to advertise itself all over Google properties for free, and would need to find ways to fund itself. They could strike their own search deals, or maybe even start charging users. Either way, they'd be on a much more level playing field with e.g. Mozilla.