I think he was right, IIRC him saying that, because no matter how great DKSII can be, it cannot have the key element that made DKS and DES great, which is the joy of learning. They can give us more of the same and clean things up and move things around a little, which will be fine, but it will be a let down to anymore expecting more.
If they drastically change the formula but not the name, things will just not seem right.
I get that DKS is just a more realized version of DES, but if DKSII continues the trend the series will destroy itself for profit.
I think Miyazaki has the wisdom to know when to let the party end, good things can't go on forever. I bet the next title he directs will be as stunning as DKS/DES, but to be so it has to be something new.