That would be changing in future when they roll out Page Experience group as a ranking factor which includes Core Web Vital Metrics https://webmasters.googleblog.com/2020/05/evaluating-page-experience.html
We’re combining the signals derived from Core Web Vitals with our existing Search signals for page experience, including mobile-friendliness, safe-browsing, HTTPS-security, and intrusive interstitial guidelines, to provide a holistic picture of page experience. Because we continue to work on identifying and measuring aspects of page experience, we plan to incorporate more page experience signals on a yearly basis to both further align with evolving user expectations and increase the aspects of user experience that we can measure.
When we roll out the page experience ranking update, we will also update the eligibility criteria for the Top Stories experience. AMP will no longer be necessary for stories to be featured in Top Stories on mobile; it will be open to any page. Alongside this change, page experience will become a ranking factor in Top Stories, in addition to the many factors assessed.
So if 2 web sites on same topic have same SEO juice and if one’s page experience i.e. page speed is faster than the other, then the faster site will rank higher regardless of if it’s using AMP. I’ve already seen site and even forums in Google News that do not use AMP so it is starting to change.
Though I still agree, separate caches for AMP ?amp and non-AMP pages is necessary.
Though whether you want to use AMP will eventually depend on overall page speed as in future Google page experience rank will probably not care if you use AMP or not. I’ve seen AMP pages with 32+ seconds page load time where all 32+ seconds had 100% cpu utilisation on the web browser with 1,200 requests on the single page load. Google isn’t going to rank that site with AMP and 32+ seconds page load time higher than a similar site on same topic that loads with non-AMP page in <2 seconds on mobile.
Basically, AMP only makes sense when AMP pages are faster than your non-AMP pages.