It's hard to say, I'd say it would be more of a case by case basis, ie...if you really like the sound of a certain DAC, and what that sound brings to your system...old or new is a non issue.
My old Counterpoint DAC and Kinergetics transport (circa 1992 or so) sound every bit as good as two $3,000 cd players I've owned (at least to my ear).
So, I guess I'd say...on a low-mid level budget, no...newer is not always better. I would expect that the new top level ($$$) DAC's are indeed better in every way ...just like the old DAC's were in their day.
Dave