Let's be real here, a lot of rappers rap with substance some of which post on this very fukking website like :
@AsAboveSoBelow
@Kaedus
and
@
ℒee
By virtue of being rappers period, they're already apart of a certain niche.
Changing their subject matter in no way guarantees they'll reach a broader demographic or that even if they do that broader demographic will mess with their music.
If anything more often than not a "broad subject matter" ironically condemns a lot of these artists to low sales
and indie circuit careers UNLESS they've got the machine behind them.
Skyzoo for example is easily lyrically as nice as Lupe fiasco with the pen but he doesn't have the machine behind him like Lupe did thus he could never hope to get the spins necessary to reach a broader audience.
So that means, he'll never get in the ears of people who only listen to the radio or watch televised performances or follow the grammy's. Speaking of which
Macklemore's image is considerably more easy to swallow than Murs to some people but content wise you couldn't tell me Murs hasn't touched on topics like Homosexuality, infidelity, urban violence, race in Hip Hop music etc. He's just as dynamic and complex an artist but one guy didn't blow up and become the "it's okay for me to rap" darling of the larger (white...) american culture.
Let's not even touch on "young" or "old" demographics.
You try selling Danny Brown to a guy who only likes Led Zeppelin or Luther Vandross and you've lost already.