I only skimmed this thread, so hopefully I won't be redundant, but let me give a few musings over why I really hate today's rap music (and why in hindsight I seem to appreciate the oldschool stuff more) when it comes to sampling
Aw. You didn't say anything about how the predominant use for rapping has gone down the gangsta toilet. I hope one day we can look back and consider this guns-bitches-and-bling stuff its own distinct genre of oozing arse instead of as just 'rap'.
But yeah, compare the Bomb Squad's work on classic Public Enemy to the sort of stuff that's around now.. huge difference. (Also Chuck D's voice could bring down buildings.) But ages back it was much easier (and cheaper!) to sample. People didn't consider it a revenue stream the way they do now; it was still a novel enough recontextualisation when done right that it delighted people to hear old chunks of music in new settings which enhanced it. Then they realised "hey, we're rights holders! a) we can get a piece of the action too! or b) wahh we're being ripped off SUE SUE SUE" and that ended the free-for-all. Aw.