No dude, it isn't arguable. The fact of the matter is Kanye isn't even on the top 100 list of best selling artist in the US. He isn't in the top 10 of best selling rappers. He has never been a top selling rapper of any year. Kanye's best selling year was last year, and Lil Wayne stomped a mud hole in his ass selling twice as many as Kanye.
Human beings are so flawed in the lack of perspective we have. Whatever is cool now seems like he was always cool and will be cool forever. It isn't very hard for a musician to be good in the "now". There are thousands of musicians who are good now. 99.9% of them will be has beens that hardly anybody gives a shit about in 5 to 10 years.
Kanye has to distinguish himself from the rest, and in 10 years you can make the argument that he is a genius, and in 20 you can make the arguement that he is the grestest rapper of all times.
Personally I think its funny that the best selling rapper of the decade is a white boy.
At the moment, he is on the B list of all time greatest rappers. I believe Kanye isn't in the same playing field as rappers like Eminem, Tupac, or Jay Z.