No, you are wrong. Scientists have proven time and time again that there are different muscle fiber types:
type I - slow-twitch
type IIa - fast-twitch intermediate
type IIx - fast-twitch explosive
Each runner has a different setup of fibers. Muscle fiber distribution is just ONE aspect of running performance, yes all elite middle-distance runners are "fast", as in they are all ~3:30 guys, but they still have big variations in muscle fiber distribution.
There are many HS kids that are always the fastest on 100m reps, or short hill sprints despite doing no sprint training (more fast-twitch %). Then there are the ones who run crazy XC times despite doing low mileage (slow-twitch monsters, think about Justyn Knight who ran low 14's off 40 mpw).
Take two competitive recreational runners, both have a PR of 32:00 in the 10k - but one has a 400m speed of 54, and one a 400m speed of 60. Who is going to win a race in 34:00 when it comes down to a sprint/kick at the end?
Same on elite level. Carlos Lopes and Fernando Mamede were both 27 min 10k runners and the very best in the world. Lopes only had 54s 400m speed, and Mamede had 47s 400m speed. Their training was completely different, yet in a 10k race, both were extremely strong and set records.
Same for current athletes. Lewandowski in the 1500 has 47s 400m speed, whereas Ingebrigtsen only has 50s 400m speed at best. Lewandowski upset Ingebrigtsen in EU Indoor this year, by waiting until 200m to go and then easily passing him and just running his own pace. Even tho both are great, talented runners, the muscle fiber distribution of Lewandowski gave him the edge in that slow race.
Why is this important? Cause the training is very different too. Both run 100 mpw, but Ingebrigtsen does much more work on threshold/stamina, whereas Lewandowski does many fast reps, going down to 50s 400m pace in his workouts (there is a video of him doing a workout on the track which consisted of several ridiculously fast reps). He has more intensity in his training, because that's the stimulus he needs for his muscle fiber setup and running characteristics/strengths.
Yes, I agree with you that sprinters could become decent at longer distances. Carl Lewis and Bolt could both run 15 min 5k if they really wanted to. But how would you train them? Doing long runs, or distance runs wouldn't work, it would feel so awkward to them that they would quit after not even a mile. So you prescribe them lots of 100m or 200m reps, at 50% effort with walking recoveries. Then, you reduce the recovery and work on their aerobic abilities different than for someone with lots of natural slow-twitch muscles.