Apple don't keep other browsers off to keep electron out or make things safer, it is to ensure they have complete control of the web platform, which otherwise is a viable alternative to their app store, just the sort of alternative you're suggesting in fact.
Capitalism needs regulation to work well, if unregulated, it very quickly gives rise to robber barons and bullying of both consumers and smaller companies. Unfettered capitalism leads to monopolies, coercion and rent-seeking.
FWIW I’m all for regulating the hell out of anything with a corporate charter. As far as I’m concerned the deal they made when they asked us to let them have the privileges of incorporation is that we can do whatever we like to them, should we decide it’s in our interest, and if they don’t like it no-one’s forcing them to keep those protections. Half the reason I’m so keen on iOS‘ particular model of software distribution to begin with is because we haven’t regulated massive-scale collection of personal data out of existence. Give me that and I’ll join you to burn the last iPhone on a bonfire, or whatever. That’d be wonderful.
Except the browser thing—that's already fine IMO, and I think keeping other browser engines off is very nice because it keeps the Electron-type riff-raff out of the store, among other reasons, and besides it's just the browser engine that's restricted. Again, what I'd rather see is another iOS-like OS & platform come on the market, also only one browser engine allowed, but for that browser engine to be better than Mobile WebKit, forcing Apple to improve or at least giving me another option in the same product category as i-devices, but with a browser engine I like better.