actually, according to the Drum Corps World history books, I do believe the actual peak to be about 5 years before DCI, but the drop off didnt really become so drastic til the 70's.
no, it isn't all DCI's fault. Granted, like it or not, they did play a part in it, but it can't all be blamed on them.
whats sad is in the early 90s, you see some growth again, only to see it die down.