Nas has never really been that mainstream tbh.
Nas stans are in denial towards that fact.
He reached his peak in 94-96 and he hasn’t been able to reclaim that spotlight (except during the height of his Jay beef in 2001).
Eh...that depends on what you mean by "mainstream."
Nas has numerous Platinum albums. And, before Street's Disciple, he had no problem getting radio airplay.
NY State Of Mind still gets played in TV commercials today and is a classic staple in hip hop.
Nas obviously wasn't as mainstream as Jay, Pac, Biggie, 50, Ye, Wayne, Kendrick, etc. but he still garnered enough mainstream success in 94-03 to label him a mainstream rapper.