Michael:
I think the biggest difference between old and new courses is not the macro-scale contouring, but the micro-scale stuff.
For the most part, the classic architects didn't have as difficult sites as some which are built on today. They may have had a few severe topo features within a site, but they designed around them as much as possible, the same as a lot of us do today.
However, the standard of "finish work" was far different in the old days, when everything was done by hand labor. There are a lot more wrinkly contours in the fairways, around the greens, and even on the greens of the old courses because they were finished at the scale of a man with a rake and shovel, not with a D-4. You CAN create those same kinds of contours with equipment, but it's easier not to, and most people today would consider those sorts of quirks "bad finish work" which leads to a poor reputation for your company.
I think a bit of it is golf cart-related, too ... some of the older fairways are bumpy to ride on in a cart.