Did I miss the part where the average golfer all of sudden became a 1 handicap hitting 300 yard drives?
Nope. However (1) the ideal golf course design provides an interesting challenge for all levels of golfers and (2) at any club with a large number of young and/or scratch players there will be people with swing speeds close to PGA Tour level.
I will never forget standing on the tee of a 350 yard par 4 at a local muni in an evening league and having one of the players announce that he was going to wait for the green to clear before teeing off. I was annoyed - the guy was not that good and play was slow. The green cleared and his tee ball landed in the middle of the green and went over. Good thing he waited.
The spread in tour player v. average player driving distance has increased from about 50 yards in 1980 to about 100 yards today. That spread makes achieving an ideal design much more difficult. A 400 yard par 4 might have been a driver mid-iron in 1980 and be reachable for the 18 handicapper. In order to have a driver/7 iron for a the average tour pro today, you probably need something like 470 yards (just guessing).
Many courses have added significant length over the last 20 years, presumably because they want to host events or because they need to do so in order to charge a premium as a "championship" course. Added length has impacted both classic and modern courses.
My club was built in 2000 and hosts a college tournament. Originally it was about 7100 yards. Now, the back markers measure over 7500 yards and can be stretched another 100 or so. Top end classic courses in the area have also added length, potentially compromising design quality in some cases.
Tom Doak has expressed his view that one should just forget about the long hitters and build a course for the vast majority. I am fine with that approach and think it makes logical sense for 99% of courses. However, courses continue to lengthen. I am sure part of that is due to ego, however, I do not discount the possibility that courses of that length are able to generate more revenue because they are perceived as capable of challenging the best.
This is a long way of saying that I believe monitoring driving distance is an important issue for architecture. I think it is important regardless of the cause of increased distance and regardless of whether one believes additional distance is a good or a bad thing.