"Setting such long expiry times on the cache also sounds like a great way to pollute browser caches with obsolete resources. "
Setting "forever" caching is standard practice for web development for many years. The browser doesn't really cache these forever - it is merely a hint to the browser that the resource won't change and you don't have to check it again.
In fact, the more resources you can put with such caching the better both for you and the browser's speed.
You really can't control how long something stays in a browser cache. The user can clear it at any time. Mine are cleared every time I close the browser.
Like I said the header isn't to control how long something stays in the cache. It's to control how often the browser needs to check if the content changed.
Setting "forever" caching is standard practice for web development for many years. The browser doesn't really cache these forever - it is merely a hint to the browser that the resource won't change and you don't have to check it again.
In fact, the more resources you can put with such caching the better both for you and the browser's speed.