B.S. That's the kind of thinking that puts so many athletes behind - kind of like NOT running a Marathon until you've peaked at 10000m, no wonder there are so few great Marathoners these day in America -let's all wait until we are over the hill!. It also makes for a short career at what may be one's best distance. I happen to think that Geb and Tergat waited too long if they wanted to maximize their potential in the Marathon - with their 10000m capacity they might have been running 2h02 or faster if sufficiently prepared for the distance when they were younger and running their best at 10000m. I can't blame them however (especially Geb) as they were acheiving good success on the track, so why try and do it all at once.
As to middle-distance, Cram (if you look at his progression, and he did run a faster mile and 1500m than Coe) alternated progressing his 800m AND 1500m for years until he peaked. I believe that if Cram had done the specific Strength work (Hills, Plios and weights) that El G has done then he would have run faster (possibly 3.26). His was a sensible program based on endurance and speed-endurance otherwise. A balanced development around all required energy systems is the best - then you don't need to move up at all because you maximize your potential at your best distance. The trick is to determine what that "best" distance is for you. Also, your idea assumes that speed development is maintained from below as you go longer - maybe, but not necessarily. If you sustain a long-term injury (like a year) that this mean that you have to start over again?
As for peaking, you seem to think that it just happens as "due course", and that's it - again B.S. - there is nothing to say that you can't get better (beyond age) in an event - although it is true that few middle-distance runners (800/1500m) actually significantly improve after 25. But choosing to move up may be sensible at that time as making improvements may get more difficult (although not impossible)and the opportunity to improve at an untried distance (assuming sufficient aerobic background)provides motivation to "move up" for many athletes.
Coe is certainly not a good example of your idea as he spent his formative (high school) years running 3000m races as well as 400m's. In fact he moved down during university to focus on the 400/800m for a while, more because he saw a weakness that needed correcting - it had nothing to do with peaking.