it is possible as in technically but it is not cost effective. the price of 64gb flash chips is far too cost prohibitive for apple's profit margins. if they did make one, it would sell for about 600-700 dollars so nobody would possibly buy it.
The iPod classic is a bad-ass little workhorse. No shame in owning one. I have one, and I'll keep it regardless of what Touch I have. You just can't beat that massive amount of storage to keep anything and everything on it at all times.