This subject interested me as a front end developer (not much php experience but tons of js). I looked at pretty much every component I have (63) and saw that there is no overlap in js libraries except in one component developed by a standard member here, not core team, that has unnecessary full library file as well as minified file. People could be irresponsible and create components duplicating libraries that are already in the core or choosing a non-minified file over a minified version but surprisingly with all the different authors of my components (core, premium, private, and personal) this utilization is pretty good. I know components are only a part of the equation, I just jumped there first because I thought for sure I would see overlaps and unnecessary usage. Anyways, just thought that was interesting.
Re: gutting bootstrap file to only what is required or any other large framework for that matter, good luck not breaking something let alone creating a maintenance nightmare for yourself as you update versions. This places all of the ownership on you instead of dropping framework owned files and just doing regression testing. This may be just my opinion but at work if I see someone modifying a library file I will spank them and put their nose in the corner.