You may be right, but if we back away from the idea of good athletes becoming great simply by adding mileage and instead look at it from the perspective of any athlete becoming better the conversation becomes more interesting, especially at the high school level (or maybe that is just the angle that interests me because I coach at the high school level). There are athletes who are built to be sprinters and adding mileage for the purpose of moving to a longer event will not produce any benefits for that athlete. We have had some sprinters in that category join cross country and then return to sprints. Some have become significantly better at sprinting despite being horrible cross country runners (even though their attitudes were good). So that leads to a few questions.
1) Was it really that cross country made them better at sprinting or was there another explanation (like a year of growth)?
2) If cross country did make them better, was it simply because they would have been on the couch playing video games and something is better than nothing, or was cross country the best way to elicit some of the adaptations that helped them? If it comes down to something is better than nothing, would they improved the same amount by regularly playing tennis or some other active, but non-sprinting activity? Would they have improved by even more by participating in a program built on anaerobic stimuli that were both lactic and alactic in nature?
On the other side of it, if you were to go to most high school track practices and look at the school's sprint group (including the school where I coach), you will see a lot of kids who clearly are not built to sprint and would have greater success in some other event group. Some of those sprinters are pretty good high school sprinters, but could likely have more success in another event group. Some are kids who are slow and will probably be slow in any event group. But an athlete who is a good high school sprinter, might make a good college 800m runner or more. A kid who is a slow high school sprinter might make a more serviceable distance runner. Obviously there are more event groups than distance and sprints, so the same logic could apply to those event groups also. It sometimes takes a while to get a sprinter over to the throws group too. Sometimes these athletes have no desire to do anything except to sprint. Sometimes they are good enough at their event that the sprint coach makes a problem when they leave for another group, or makes them feel guilty to leave. Sometimes the athlete is so bad that the distance coach doesn't bother to try to bring them over. But there are lots of athletes in high school track and field who are not training the way that would lead them to the most success.