With real web pages, the native sharing UI just works as it does everywhere else.
With AMP, you have to know to tap a different unfamiliar icon and then know that the unstyled URL displayed is actually a link you can interact with. That means that sharing goes from one tap to 2-3 and that's after people learn that Google gives substandard results doing what they're familiar with and so you need to remember to do something else only for pages which you found on Google.
An AMP page served without cache has to be treated the same, or this becomed exactly the freedom issue that this entire discussion is about.
My pages are actually optimized a lot, and they actually get about 10x slower with AMP than without.
I don't want to have to choose between fast loading and good SEO, I want both. And Google only offering good SEO to those who let Google track all user interactions with their page is also not ideal (I specifically do not use any ads, analytics, or tracking, not even storing IPs in the server logs).
The other thing is that for me, AMP has not been faster many times. I know they're working on it but having to download and run 100KB of external JavaScript before rendering frequently meant that it was slower than the real page. It helps the stragglers but e.g. WaPo or NYT load just as fast and that should be enough.