CDNs are not magic and there's a lot more than bandwidth to consider: you still have the overhead of doing DNS lookups, additional server connections and response latency before bandwidth enters the picture and the client overhead of decompressing, parsing and processing the response, where CSS will block rendering until it transfers and JavaScript will not only block rendering but also further loads until it finishes executing. Latency is almost always far more important to the user experience than the actual bandwidth and, particularly if you have many users on wireless networks, with blocking resources the concern isn't the average or best-case latency but the worst-case.
Libraries are still usually a reasonable trade for the reasons you mentioned but it puts a solid limit on your site's performance. If most of your users won't execute most of that code, it makes sense to reconsider – for example, if you could replace most of your jQuery usage with APIs like e.g. querySelector/querySelectorAll and your site only supports IE8+, it's quite reasonable to question whether you're getting enough benefit to be worth that download or whether there's a way to defer anything which isn't used all of the time. For example, much as I love LeafletJS, I'll toss in a rel=prefetch header for it but won't load the script until I need to display a map.
Libraries are still usually a reasonable trade for the reasons you mentioned but it puts a solid limit on your site's performance. If most of your users won't execute most of that code, it makes sense to reconsider – for example, if you could replace most of your jQuery usage with APIs like e.g. querySelector/querySelectorAll and your site only supports IE8+, it's quite reasonable to question whether you're getting enough benefit to be worth that download or whether there's a way to defer anything which isn't used all of the time. For example, much as I love LeafletJS, I'll toss in a rel=prefetch header for it but won't load the script until I need to display a map.