Great "generational divide" story :)
As a senior...(cough cough) I can't help wondering how often you remind your older squash partner about his slower pace /bad knee etc etc

This is (in many ways) like saying "I hope all you retired folks have a bad 10 years in the market so us youngsters can invest more and then  clean up from 2026  to 2040...."

As for predicting future returns over the next 5/10 years /20 yrs etc (to somehow play out in younger investors favor) ....this is just rubbish.
I will give you credit for the "cost averaging and contributions" angle.We geezers can't have everything...and we know it :)

IMO any cagey investor who does not need income ++ (at any age) would have a lot of cash right now,
(be selling,as you say when it is high) recognizing that,yes,the market is extremely high....
This maneuver (which is free btw) would be a much more effective action than dreaming of some future time of "golden bounty"....
Index investing....Bah!