You're right that Nas, Hov, etc came up in an era when access to studio time was so expensive and restricted that you had to be better than average to get a good shot. I remember Large Professor saying that he would bring Nas to the studio back when he'd produce for Eric B & Rakim. At the time Nas was nobody, and when he'd try to speak with Rakim he got straight up dissed/ignored. There was definitely a hierarchy at the time, whereas now anyone can get an iMac or find someone at school who has one, download some beats off soundcloud, and record a song. This leads to a very diverse group of people making music, but also a lot of wack shit.
Which brings me to something that's missing today: the A&R. There was a time when every label had someone going to clubs, going to corners, going to high schools, etc...scouting talent, aggressively. Whereas today labels are quite reactive: some dude gets x amount of hits on soundcloud, or gets x amount of youtube videos, and a label calls. Someone gets a big internet hit song and a label wants to sign you. Very little actual relationships are made, research isn't being done, etc. So we get guys like Bobby Shmurda, who literally releases one official song and gets a deal. Epic will eat off Hot nicca digital sales, delay his album to oblivion, it'll sell 4k first week, then a year later he'll be dropped after a subpar tour (during which they'll get a % of his booking money). It's nothing more than a machine churning out meat for rather subpar profit margins.