I agree that loss of elasticity can be one of the limiting factors as you age.
Loss of bounce and springiness has hit me very hard the last five years, and I believe it is the primary reason for my personal recent rapid running slowdown.
My theory is that loss of muscle/tendon elasticity and scar tissue buildup from micro tears in muscles now severely limits elastic energy return in my stride. Not sure if that is reversible.
How I reached this point follows but is not typical.
I am 60 and have been running 70+ miles a week for 45+ years. Over 120,000 lifetime miles. PRs 4:12 mile, 30:50 10k , 2:28.marathon
Same weight and motivation as when young. Some strength declines, but not that much. Some flexibility declines but not huge. Minimal leg speed declines from when young. No injuries.
Essentially it feels like I get very little elastic energy return from muscles and tendons anymore. Very noticeable when running downhill, as the free speed gain from bouncing downhill with gravity is not all there anymore. Legs often just feel dead.
I am not a plodder/shuffler (yet) but it takes a lot more effort and work to run the same paces without elastic energy return.
Yes I have worked hard to limit/reverse this in the last 5 years with very little success. I do more plyos, stretching, strides, shorter intervals, hill sprints, weights more than ever before.
With limited time I've cut overall mileage 40% to accommodate this. Sometimes I think focusing 80% on drills and such, and cutting miles to less than 20 a week might help. But I would not enjoy being a gym rat instead of a runner.
My lifetime of running involved lots of relatively slow but hard and hilly trail runs with significant ascent/descent at high altitude. An easy 10 miler might have once taken 85 minutes and included 1500 feet of climb with the same descent at 9000' elevation.
Lots of sore quads after hilly mountain/trail races (delayed muscular soreness) Lots of pounding at 168 lbs.