To clarify, the focus of a program should always be a balanced approach. I am never discounting the other aspects that those of you have mentioned. They are certainly important and 100% included.
To hit some of the questions:
1) How do you develop limb speed?
There has to be a focus on it. I think that Pat Connoly had it down with Evelyn Ashford in her shake-ups. The volume was a little high . . . but the emphasis of building up the rate of stride as you move down the track had it benefits. It was basically 100m of tempo with every 20 meters the stride rate increasing.
Also, a lot of coaches use stick drills or cones drills to hone in on this skill. Some daily, some alternate, but they are done often.
Then the obvious one is sprinting.
2) Why couldn’t one focus on those late in the career (limb speed and elasticity)
Sure you can. If done earlier, the athlete will have far greater success in building it. Think about 20 years of limb speed development vs. 2 years. I also think it is important to build strong tendons early with jumping rope, skipping, and things of that nature.
3) Force production.
Most people know that producing force on the front side is 2x greater than producing force on the back side. Also, it is easier to apply more force with greater speed. Think about shot put moving at 5mph v 30 mph. So by developing limb speed, you are automatically going to produce more force as a by product of the speed of the down stroke.
By developing stronger, faster hip flexors, the athlete is able to maintain position better, longer and hit more front side steps during the race. We all know that force is a vector so it is not just how much but the direction it is being applied.
The best sprinters in the world use physics to the their advantage by holding their front side mechanics. They are able to create lower leg rotational speed towards the track and nearly the match the speed that they are moving forward. This make it easier to apply force because you are not wasting time and effort catching up.
There are many, many factors to consider when developing a complete balanced program. I just happen to think that a little more emphasis on stride rate and developing powerful hip flexors pays bigger dividends.