I don't think I agree with Marsalis' theory exactly. Although I'm not entirely sure what it is, although, like him I come from a classical background
Maybe to him a music hater was a bit harsh, but he's an uber-traditionalist, he was a vocal critic of Miles Davis more experimental latter day work, and he completely dismisses hip hop altogether. I do actually love his love for old music, but the man got trapped in the past.
When we look at rap, I laugh at the thought that it's going someplace. I've been in that industry and watched the fall. The 1 or 2 actual great artists out today aren't from today. You can cling to Kendrick Lamar as some new savior, but the truth is the only thing they lyrics today contain is vulgarity and basic word play. To even compare the content to the standard of the 90's is crazy flat out.
All this talk of the "fall" of rap is exactly the kind of hyperbole I think is total nonsense. For example there's some old conspiracy theory I read a while back that goes along lines of "In 1990 a group of country's most powerful music execs had a large meeting, and decided on a vision for the future in which they all bought stocks in private prisons while simultaneously pushing Gangsta rap to the forefront of popular culture, thus more young black men than ever will get sent to the private prisons and they're profits would be through the roof, while rap turned to vulgar shit". This is unlikely to have ever happened (at least that way), but what I'm trying to show you, is that for a previous generation rap had already "fallen" in the 90s that you laud as the peak of rap. Your no different from those guys. I'm not exactly keen on the majority of current mainstream stuff either, but guys like this make music I find exciting http://mellomusicgroup.bandcamp.com/album/niggas-is-men
Maybe not as prefect on a technical level as your KRS 1s or BDKs in their primes, but then, that's already been done, you can't get any better on that level, why not try something new, and more impressionistic? That's what many of these new cats seem to be doing. And if you do want that 90s style boom bap with a new flavour, there's still good new stuff on that tip out there too, such as people like Rac Marci, Action Bronson, Ka, even Danny Brown, and of course Joey Bada$$. So really, right now there is something out there to please everyone, basically.
the industry today tends to kill it- in the name of progress.
Stop looking towards the "industry" then. Look in more left-field sources, and you'll find plenty of what you want.