Yeah, we will have to agree to disagree. I felt Born This Way was vastly superior to King Geedorah, word play and beats wise. I think there is an overly harsh stigma on hip-hop artists, that really isn't applied in other genres. It's hard to replicate the feeling people get when a whole new door is opened but I don't think that means they are creatively bankrupt. For instance, I felt the Blackalicious (and several of Lateef and Lyrics Born solo projects) releases continued to deliver after they dropped the Sole Sides moniker. At the same time, how do you beat the impact of Latyrx dropping? It's much like when the Wu saved us from the MC Hammer era. None the less, I never see Nirvana or TOOL getting the same kind of criticism. Hell, people get so entrenched in their initial exposure to an MC, it is even a standardized joke now.

How many hip-hop heads does it take to screw in a light bulb?
three, one to screw it in and two to argue how much better the old one was

I guess I feel that rap is so reliant on lyrcis whereas "rock" isn't. Therefore a rapper runs out of interesting things to say, but a "rocker" never needed to say something interesting in order to make a good song. It's also much easier to write lyrics for "rock" than rap - cadence and rhyme structure are MUCH looser in "rock", so you have much more freedom in your song writing.
It may be a joke, but it's also generally true - a rapper's early work is almost always better than their later work. Though I don't think that's specific to rap. And there are certainly exceptions.

That's my issue with the perception though, is it accurate? Can anyone really say Aesop's flow was tighter or more complex on Earthworms and Float, then on Bazooka or Labor? Was Blueprint further along on 1988 or when he spit on Alchemy? Cage was sick in Smut Peddlers but was it better than when he dropped solo on Def Jux? I hear yah though, the MC is and always will be the crux of hip-hop. Is someones flow is tight enough though, I will like the song even if they are just clownin. For instance, Lyrics Born on I'm Just Raw was killing it without putting the word play in your face. I really feel that people like Bus Driver and Brother Ali have continued to improve their delivery and flow. It's apples and oranges at this point though. It's obvious you have good taste in hip-hop and we heads are damn picky.