I'm not quite sure what you're arguing here, but Jays discography is really inconsistent in terms of album quality.
You know how long it took for him to fall off with beat selection? Around the "Kingdom Come" era, then "BP3" still had fire beats, along with most of "MCHG", albiet they were far from his prime in terms of rapping and songwriting. The Vol Series was a natural progression leading up to the BP, then you had AG after KC, which wasn't that much of a drastic steep in quality compared to BP 3 onwards to MCHG.
Each album excluding maybe 2, all had very consistent, high quality production if you're arguing your point on music. The irony is Kanye didn't have a lot to do with that either, and was purely good timing on the music direction during that section of his career.
If you meant his emceeing and songwriting, that's harder to substantiate. His decline began after BP 2 (One disc is dope, the other is mostly filler. Overall, good album, but it's clear he did it more to be commercial than balance). Because on Vol 1-3 (all 3 varied on these aspects which is impressive in execution), he still was lyrical and technical as fukk and on BP, he balanced out his cons for the most part. He was more modern and had more of a background behind who he was, rather than relying on some outdated drug persona (Which he brought back on "American Gangster" to give more history and not exemplary bragging like on "Reasonable Doubt", so it was refreshing).
His unreleased shyt like "People's Court" as a song for one example, was fire too.
When you move on to the Black Album, the transition makes sense from the BP 2, because that was giving way to his "Retirement album", and it would have been a fitting ending to his discography.
He nearly gave you all sides of himself and touched on everything fans wanted him too throughout a 10 year period, which was outstanding. If he didn't make anymore music after TBA, he would be revered much higher than y'all are undermining him atm.
So outside of 1 bad albums, and 2 mediocre ons he dropped this decade, his 3rd decade doesn't discount what he accomplished in his first two. That is far from inconsistency as 2 of them (MCHG and BP 3) arrived at the near end of his career, while KC was a bridge from TBA to the AG. Note, he was personal on "Kingdom Come", as "American Gangster" would provide a clearer lens at the time through all the shyt he went through before making that project.
Nas meanwhile since "It Was Written", fukked up on every other album and is the result of what happens what you cannot stay in your lane and what you're good at.
On "I Am" he tried rapping fast in one instance, on "Nostradamus" he tried being some "spiritual lyrical miracle" type rapper and remaking "It Was Written". The worst part was
that nikka dropped those 2 
IN ONE YEAR. DMX and Hov pulled it off, but NOT NAS!?!?!?
"God's Son" was great, but it wasn't a return to form like he demonstrated on "Stillmatic". Then you have "Hip-Hop Is Dead", which was Nas trying to be "Real Rap", like given all this nikka did for trend hopping, he's gonna now do this

to other rappers?
When "Life Is Good" came out after that b*stardized creation that was "Street's Disciple", how is that not the textbook example of inconsistency in Rap?