Reality is that this is already happening and its already a problem for them. For instance, I would have been very capable of "taking the lead" in the congregation. I left when I was 35, but from 20 to 35, never actually reached out to become a ministerial servant as I wasn't able to teach things I didn't believe in. Other brothers who felt the same confided in me about it. The end result is that there are less brothers to take the lead. One of my uncles is an elder and has been complaining that there are simply not enough of the younger generation to take over. A portion of me finds it sad for him as he dedicated his life to a cause where he's unable to simply pass the torch, validating all of his efforts. But that's basically what is happening.
They are addressing this issue by lessening the teaching burden with videos, merging congregations, removing the book study and removing days from conventions. But the issue will keep growing.