All this common web stuff that is distributed by several CDNs (as well as separately by individuals) really suggests to me that there should some browser feature like `<script src="jquery.min.js" sha256="85556761a8800d14ced8fcd41a6b8b26bf012d44a318866c0d81a62092efd9bf" />` that would allow the browser to treat copies of the file from different CDNs as the same. (This would nicely eliminate most of the privacy concerns with third-party jQuery CDNs as well.)
So to take it to a bit of a rediculous (but still possible) point, I could probably guess what your HN user-page looks like to you. So from there I could serve that in an AJAX request to all my visitors with this content-based hash and if I get a hit from someone, I can be pretty damn sure it's you.
And that only really solves one or 2 of those issues. The versioning, compression schemes, formats, number of fonts, and sizes of browser caches will still cause this system's cache to be a revolving door, just slightly more effective.
And as for the security concerns of using a CDN. Subresource-integrity (which someone else here linked already) allows you (you being the person adding the <script> tag to your site) to say what the hash of the file you expect is, and browsers won't execute it if it doesn't match. So that lets you include 3rd party resources without fear that they will be tampered with.
Solution: Using a sha256="..." attribute should only allow you to access files that were initially loaded with a tag that has a sha256 attribute, and this attribute is only used for resources the developer considers public.
This not only solves the CDN issue but it also solve the issue of having to rename the files manually everytime someone do a change. It just makes caching that much saner.
If you see a script tag with the URL bank.com/evil.js, the browser shouldn't assume that the bank is actually hosting evil.js. Even if the hash matches, the content might not be there.
The bank might be using a content security policy to minimize the damage that an XSS attack can do. It only allows script tags from the same origin. However, now an attacker just needs to load evil.js with a particular hash into the cache, and they can create the illusion that the site is hosting it, without having to hack the server to do so.