I think most classics or so called classics are influenced quite strongly by outlets like The Source (back in the day at least). I think there's certain albums had they lauded classic by those outlets, for example The Source everybody would look at like a classic, and other albums had they panned wouldn't be looked at the same. Unfortunately a lot of people are sheep.
I think there are albums that break the mould though, there's certain albums that may not have got that rating from The Source but the people won't allow it not to be classic, if I'm correct The Chronic didn't get 5 mics originally? All Eyez On Me also? Infact NO Pac album? Lol. But the people overall said The Chronic and All Eyez On Me was classic regardless. So I think it's possible to break the mould, but more difficult and easier if it had that stamp.
That being said, Illmatic to ME is classic and deserves the 5 mics, I would have said that even if it got 4 mics though because I fukk with the album. And likewise an album like All Eyez On Me, definite 5 mics to me, along with 7 Day Theory etc despite what any rating says, Capital Punishment also, that's an album that I feel if it got the 5 mics (which I personally feel it more than deserved) would be an undisputed consensus classic, that shyt is remarkable.