"I ain't rappin' too fast, you're just listening too slow!"
Hell no, 40 shouldn't retire!!! People are entitled to their opinions, but my opinion is that 40 can still do it--he just does it a little differently on each CD (i.e. producers, features, etc). He admitted on an interview that was posted on BART last year that he likes working with different rappers from the South and experimenting with new sounds. To me, that's what makes his CDs stand out in that you never quite know what you're gonna get...
PEACE!