Mail:
To be fair, ever since share sheet became a thing, the 'default email app' has become a much less relevant concept. (i think it only really matters for mailto: links or whatever iOS parses as email address and creates a mailto link from)
Otherwise, like when sharing photos, it's functionally identical whether you hit share with -> Mail or share with -> GMAIL for example.
Browser:
The default browser, on the other hand, should be something you can choose, period.
Safari would still be many people's browser of choice due to its simplicity and its interoperability with macOS Safari.
Maps:
Opening up CarPlay to third party apps (i.e. GoogleMaps) in iOS 12 was a big deal as well. So, apart from tapping addresses in the contacts app, you really never see the Apple maps app, if you don't wanna.
Like, googling for a place's adress in Safari and then hitting "route" on the Google result
will open the GOOGLE MAPS app on your iPhone.
Can someone explain why this is okay while MS having Internet Explorer installed and as standard on Windows wasn't okay?
Market share.
Microsoft had a de-facto monopoly among consumer PCs and other "PC browsers" were (in parts) still commercial software.
iOS has like ... 25% market share among smartphones.