Past swings away from OS default browsers have hinged on the superior features, better performance, improved security, and superior site compatibility. Marketing and distribution play a prominent role but have been indecisive in recent browser battles. The leads of OS incumbents are not insurmountable because browsers are commodities with relatively low switching costs. Better products tend to win.
This is what always gets me about debates over Safari being the only allowable engine to developers on iPads and iPhones: traditionally, the best browser has won, so why wouldn’t Safari win on iPadOS and iOS as well? If the argument becomes, “if given the choice, people would choose a browser with a different engine,” then I’m not sure how strong your argument is that Safari is actually the best.
Apple's prohibition on iOS browser engine competition has drained the potential of browser choice to deliver improvements. Without the ability to differentiate on features, security, performance, privacy, and compatibility, what's to sell? A slightly different UI? That's meaningful, but identically feeble web features cap the potential of every iOS browser.
On the other hand, there is open choice for rendering engines in macOS and Windows, but every single new browser from Edge to Brave to Vivaldi to Opera to Arc all use Chromium as their rendering engine. These browser makers have chosen to differentiate on exactly the things mentioned above and it feels like we have a relatively healthy bit of choice for browsers on traditional desktop OS’s today, even though all of them but 2 (Safari and Firefox) use the same engine.
Should browser makers on iOS/iPadOS just make a better feature set and interface? Do they have the tools to make something as good and fluid as Safari? I don’t actually know.
In 2014, Apple would have enjoyed a profit margin of 50% if it had spent half a billion on browser engineering. Today, that margin would be 94-97%, depending on which figure you believe for Google's payments. In absolute terms, that's more profit than Apple makes selling Macs.
Crazy to think that even if Apple dedicated more money to maintaining WebKit and Safari than Mozilla does for their browser, they’d still make more profit than they do selling Macs thanks to the Google search deal.