The javascript is heavily cached, so will not give a request on every page load.
That is not the intention. If javascript is disabled entirely, Google Search won't even load AMP pages. The scenario you describe of a user loading an AMP page directly without javascript enabled is somewhat rare.
Many people use tools to block third party JS from loading. AMP can't be called privacy-friendly while making it extremely difficult to use when tracking (AMP Analytics) is blocked. The 8-second delay happens to me every time I accidentally click an AMP URL in my browser.
I don't use Google Search, and I frequently get sent to Google's AMP cache via other link sources (e.g. HN).
I don't have javascript blocked, but I do have Google's tracking blocked via standard tracking protection (which is now a built-in feature in most non-Google browsers), which means <noscript> tags are not triggered, and I get the 8 second delay due to non-loading JS resources.
I don't think my setup is as rare as you make out.
That is not the intention. If javascript is disabled entirely, Google Search won't even load AMP pages. The scenario you describe of a user loading an AMP page directly without javascript enabled is somewhat rare.