I think that's an over simplification based on your needs/wants. I'd love to see some improvements to the default keyboard, but I don't believe Apple should allow 3rd party apps to replace default apps.Indeed two pointless and honestly frustrating lock downs.
1: No 3rd party keyboards installed (I used to 3rd party ones on my Palm5 years before Apple)
2: No 3rd party default web browsers allowed.
Thankfully now that over controlling person had gone, Apple can move forward as a company and stop treating it's consumers like children who would not understand anything other that what he deemed they should be using.
If you want a consistent user experience you have to control all aspects of it. Nothing is more jarring to UX then drastic changes between screens. One of the main reasons people choose Apple is because of their consistency.
Apple would have to vet replacement apps to a higher degree then normal apps to ensure they worked in every situation the default ones do (other apps linking to them etc). What benefit is there to do this at the risk of increased issues? A small percentage of customers would appreciate it I'm sure, but the majority (based off the users I know) wouldn't bother. I'd rather them put effort into improving the keyboard rather then creating ways to replace it.