How fruitful would it be to make large parts of an addon (especially large static data) load on demand, or load after player entered world?
AddonLoader already does that (with support from addons), and it does indeed reduce the time you spend staring at the loading screen. However, it also causes annoying pauses in-game when large or even medium things are loaded, which is why I disable its functionality for addons big enough to cause that that I know will always be loaded at or near login anyway. Quartz was one example of this... I will never play the game for more than 30 seconds without casting something or seeing someone else cast something, so I'd rather spend another 2 seconds at the loading screen than have a 2 second freeze when I mount up. And then of course there are addons that don't support on-demand loading, which are themselves large-ish and/or load large chunks of data (*cough*Recount*cough*:p)...
several authors here have a strong opinion to not do this or provide methods for no-lib use
If that is actually going on, that is idiotic. If a library is written and released with a standalone version, it's just petty and obnoxious to refuse to take the extremely simple step of adding the name of the standalone version to your addon's OptDeps.
If you're talking about things like LibDataBroker-1.1 which is not released with a standalone version (i.e. there is no official distribution of LDB that includes a TOC file) then that library is not intended to be standalone in the first place, so it is not the responsibility/fault of the author of addons using it to provide a way for you to run it standalone.
Note that the numbers provided by Warmup will be completely irrelevant if you are using embedded libraries.
Embedded libraries will absolutely increase the time you spend staring at a loading screen. I don't know if the Curse Client supports standalone libraries, and nolib packages are not available on Curse.com, but if you download manually, just download from WoWAce and grab the nolib packages. Either way, if you make the switch, it's probably best to delete your whole AddOns folder (or rename it for reference) and do a fresh install of all addons and libraries.