I think the premise is wrong. Different sorts of rap music have long coexisted, and oftentimes it's hard to tell if any given artist is glamorizing 'thug culture' or critiquing it or some combination of the two. If you want to point out some sort of inflection point in the history of rap music I think a more meaningful one is the turn from feel good (if egocentric) rap to hard edged gangsta rap in the late 80's, which interestingly coincided with a lot of socially conscious rap.

From the second verse of Rapper's Delight (1979):

Check it out, I'm the C-A-S-A, the N-O-V-A,
And the rest is F-L-Y,
You see I go by the code of the doctor of the mix,
And these reasons I'll tell you why.
You see, I'm six foot one, and I'm tons of fun
When I dress to a T,
You see, I got more clothes than Muhammad Ali
and I dress so viciously.
I got bodyguards, I got two big cars
That definitely ain't the wack,
I got a Lincoln Continental and a sunroofed Cadillac.

This is pretty clearly a feel good song about money and girls, but in the late 80's you get songs with mixed messages.

From Children's Story (1988):

There lived a lil' boy who was misled,
by anotha lil' boy and this is what he said:
"Me, Ya, Ty, we gonna make sum cash,
robbin' old folks and makin' tha dash",
They did the job, money came with ease,
but one couldn't stop, it's like he had a disease,
He robbed another and another and a sista and her brotha,
tried to rob a man who was a D.T. undercover

Is this a sad commentary on street crime or glorification of it?

From Fuck Tha Police also from 1988:

Fuck the police comin straight from the underground
A young nigga got it bad cause I'm brown
And not the other color so police think
they have the authority to kill a minority

Clearly a socially conscious protest song right? Yet from Straight Outta Compton on the same eponymous album:

Straight outta Compton, crazy motherfucker named Ice Cube
From the gang called Niggaz With Attitudes
When I'm called off, I got a sawed off
Squeeze the trigger, and bodies are hauled off

Seems rather unapologetic about the thug life right?

The Chronic (1992) seamlessly blends social commentary, celebration of the good life (money cars hos), and glorification of the thug life.

Let Me Ride:

Creepin' down the back street on Deez
I got my glock cocked cuz niggaz want these
Now soon as I said it, seems I got sweated
By some nigga with a tech 9 tryin' to take mine
ya wanna make noise, make noise
I make a phone call my niggaz comin' like the Gotti boys
bodies bein' found on Greenleaf
with their fuckin heads cut off, motherfucker i'm Dre
so listen to the play-by-play, day-by-day
rollin' in my '4 with 16 switches
And got sounds for the bitches, clockin' all the riches
Got the hollow points for the snitches
So would you just walk on by, cuz I'm too hard to lift
and no this ain't Aerosmith
It's the motherfuckin D-R-E, from the CPT
on a ryhmin' spree, a straight G

Lil Ghetto Boy:

Wake up, jumped out my bed
Hung in a 2 man cell wit my homie Lil 1/2 Dead
Murder was the case that they gave me
Dear God, I wonder can you save me
I'm only 18, so I'm a young buck
It's a ride, if I don't scrap, I'm getting stuck
But that's the life of a G, I guess
Ese's way deep, shanked two in they chest
Bests run 'cause brothers is dropping quicker
Ugn, too late, damn, down goes another nigga
Bouncing off the walls, throwing them dogs
Getting a rep as a young hog
It ain't nuttin like the street life
Betta be strapped wit yo clip, cuz ain't no fist fight
So I guess I gots ta handle mine
Since I did the crime, I gots ta do my time 

Around the same time gangsta rap was taking off on the west coast (actually a little before) Public Enemy was gaining traction with transparently politically conscious (but still highly self confident) music. From Bring Tha Noise (1988):

Never badder than bad 'cause the brother is madder than mad
At the fact that's corrupt like a senator
Soul on a roll, but you treat it like soap on a rope
'Cause the beats in the lines are so dope
Listen for lessons I'm saying inside music 
That the critics are all blasting me for
They'll never care for the brothers and sisters
Now across the country has us up for the war
We got to demonstrate, come on now
They're gonna have to wait 'til we get it right
Radio stations, I question their blackness
They call themselves black, but we'll see if they'll play this

Since then there's always been a wide range of subjects in rap music, with some artists finding success by negotiating a delicate balance (Notably Nas and Tupac during the mid 90's). I'd argue this variety never went away but that would be breaking the 20 year rule.