Joe, a few follow-up thoughts:

1) Your comment reminds me of something my Mega Reinvention 2016 Keynote speaker, Jim Lee, said in his presentation on The Future of Work. To paraphrase Jim, since Boomers -- the oldest generation in the workforce -- feel younger and are healthier than the generation preceding them, they feel they can work longer. Couple this with the fact that many of them don't think they have enough savings to actually support a longer retired life, they also feel they NEED to work longer. So they don't leave their long-standing positions at companies, which leaves very little opportunity for advancement among younger generations. As you noted, younger employees then leave for greener pastures.  In the case of Gen Xrs, Jim said, many of them are leaving corporate America for entrepreneurial endeavors, and this flight is going to leave companies with a big doughnut hole of a brain drain when Boomers finally do decide to leave.

2) Take a look at this great infographic, The Generations in the Workplace:  https://pbs.twimg.com/media/CGZHzLpUcAEmHgQ.jpg.  It really does also support your remarks/what companies ought to be concerned about.