Late wrote:
V, you are a D1 coach. With lots of experience.
If you prescribe your athletes all-out sprints, YOU (or a sprint coach or someone who is an expert about sprinting form and mechancis) is watching your athletes do them, actively coaching them, and correcting form/mistakes.
You may prescribe these intense sprint workouts to your D1 athletes, done with supervision (at least in the beginning), but prescribing it to a 15-year old kid, or a "recreational" middle/long distance runner is playing with fire.
How much have you worked with non D1-runners? A lot of distance runner might start doing your workouts (which are proven to work on D1 level), force the speed, form breaks down, they are straining and if done multiple weeks in a row get injured. I would never trust myself to do your workouts except the hill sprints which I consider a lot safer and I've done before, although I don't think they helped nearly as much as slightly longer hills of 20s-40s at 800-1500 pace which build immense leg strength (and don't make me dizzy like 6-8x8s all out hills).
No 15-year old in the world, has a maxed out aerobic system. That's still the main generator of energy and performance, and training that might lead to injuries needs to be extremely carefully introduced, to not lead to injuries which would interfere with the development of the aerobic system.
And my last point is still the same - sprinting and max speed is highly genetic. Some have it, some don't. Many of your athletes who have natural speed will be faster without any sprint training than the non-speed athletes could ever be with lots of sprint training, yet in a 1500m race they could both be competitive and even win depending on the circumstances.
I'm not saying that these short all-out sprints don't have a time and place, like you said they have been proven to work for decades. But there is a difference in coaching elite runners or D1 runners with supervision and instant feedback and prescribing a hard, all-out weekly sprint effort to a kid in a local running club or a self-training recreational runner (which most are).
A lot of good points, I'll address a few of them that stick out and we can see where that leads. My work with HS athletes is pretty minimal I'll admit. I volunteered for my high school program while I was in college on break so it was mainly just summer and winter that I could do much with them.
The reason I use short sprints today came from my training when I was 14. I played soccer when I was younger and wanted to try out for the HS team. I knew I wasn't the best soccer player but I was a decent runner in middle school (5:21 for the 1600m). My problem was I was pretty slow, 80s/400m was kind of my only gear. I think I got outckicked in ever single 800m and 1600m I ever ran in 8th grade (my 800m was 2:34 if i remember correctly so not much better).
Going into soccer I was pretty paranoid that everyone would be faster than me. Having only played rec soccer I felt being athletic was going to be my only way to stand out. That summer, sure I practices a lot of soccer at home, but I ran 40 yard sprints 2-3 times a week the whole summer. Since I wasn't racing I couldn't really measure if it was helping. By the time I hit indoor track I was running low 4:50's and finally having a kick that could win races. By the end of spring I ran 4:34. I was pretty happy with that as a freshman,
Granted, I had better training overall, but going from 8th grade where my fastest lap was a 79 and my slowest was an 81/82 to a 9th grader being able to close a race in 63-64 was a huge step for me and was a tool i never had until i actually practiced using that gear.
I didn't have supervision and I think most athletes with multi-sport backgrounds deserve a little more credit than we sometimes give them. I do have some runners on my team now (even at DI level) that I know can't sprint properly, The majority of the time it's the ones that haven't been asked to do it for years before they get to me. They come from the same programs that like to say, 'you'll never run that fast in a race so why practice it here?" The ones that sprint occasionally or played ultimate frisbee after practice, or were in other sports, they're usually fine. The others, they eventually figure it out.
If you're worried about mechanics, hills are a great solution. It naturally fixes a lot and has much less risk of injury. Start steep where mechanics are different enough that they don't resort to those more ingrained bad habits, then gradually make them shallower until they can sprint well on flat surfaces.