The idea is bad: here, firstly, the average Internet user, it turns out, will download a copy of each of the popular libraries to itself, without missing a single update, even if it never takes advantage of it.
While with popular CDNs from Google, Yandex, Cloudflare and the producers of these libraries, only the really needed library is downloaded when it is needed. And, importantly, it remains in the cache only of the browser where it is used.
Secondly, not every website / page developer will provide a copy of the necessary library built into the browser, as a priority option, which allows you to download it from the CDN.
In total, you will benefit from your proposal: saving time when loading pages containing popular libraries and providing a “new” feature of browsers. For example, jQuery with Google CDN I now loaded in 70ms - they would be saved.
Lose : extra traffic when updating the browser.
Those. there we win milliseconds, we lose there. Whether the game is worth the candle depends on the overall picture of use. Theoretically, a situation is possible when the win exceeds the costs. But definitely not 10 times or more (my assessment on the eye). Therefore, I - against.