The dangerous thing to me was not the email, it was asserting the misunderstanding that additional years results in inutility.

"Up or out" doesn't have to mean up the org chart. It can mean up your value through mastery, which you clearly feel you have within a set of disciplines.

It took some number of years to become "the best" at those things you described being good at. Whatever activities you were doing those years that made you the best, keep doing them … applied to adjacencies.

Later in life, after people have been good at some things for a while, it's an ego boost. They forget what it felt like to be not good at things, and that that was OK. They become inhibited from learning by both ego pain of doing something badly, and mental pain of reformatting brain and behaviors to fit in new learning.

If at 40 you have 20 years of getting good at things, at 60 you can have another 20 years of getting good at additional things. Innovation and mastery of systems comes from multi-disciplinary, multi-system understanding. The additional years can make you more useful, not less.

Arguably, it boils down to: do you like learning, and do you like what you do. If yes to both, there's no reason you can't keep accumulating ability to deliver value.