Carl, while I am not a great defender of the USGA, your comment would be accurate for any system that tries to account for the myriad of factors that go into determining the difficulty of golf courses. Consistency in application is critical. The rater handbook provides detailed standards. The issue is in training. In Chicago, we have extensive training programs for new raters. They are given exams and rating is done by teams with experienced raters teamed with newer raters so that there is an opportunity to learn from experience. The goal is consistency. I do not qualify as an expert rater, so this is not about me, but I have invited some cynics to observe ratings and invariably their attitudes change for the better after the experience. I am not suggesting that your observations are incorrect, merely that they are less the result of the system and more a result of its implementation largely caused by inadequate training and/or lack of volunteers.
It seems to me that this is the key point. The goal is for raters to do the same thing every time they rate a course, so that the differences between tees on one course, or for ratings from one course to the other are consistent. The entire purpose of the course rating system and the handicap system demands consistency from one course to the next, and from one golfer to the next, and constantly changing the standards works against that.
To whatever extent the 250 average is no longer accurate for a scratch golfer, I'm not sure I see why that is a pertinent problem. The key part of the definition is that the player is able to play to a zero index on any rated golf course, and since that zero index is based on course differentials rather than par, why does it matter if the imaginary scratch golfer averages 285 instead of 250, especially given that the second part of that portion of the definition says "CAN reach a 470 yard hole in two shots at sea level" (my caps).
This, of course, does NOT mean that the second shot ends up ON the green, much less that birdie is a result; Tour pros, with plus indexes of multiple strokes, hit approx. 2 out of 3 greens from that distance, so I think we could assume that the scratch golfer would be hitting the green and putting for birdie far less often. So whether the second shot for the scratch golfer is from 220 (based on a 250 drive average) or from 185 (based on a 285 average drive) I think we could guess that, in either case, most of the time the scratch golfer has to hit a third shot before their first putt.
All of this seems like splitting hairs to no great end; the goal is still consistency of application, rather than constantly changing standards.