I think the short answer here, or the inverse of the answer :), is that you begin to become irrelevant when engineering stops being the thing you like to do. I don't think you can force yourself to remain a valuable engineer if you don't like what you're doing, and conversely if you do like what you're doing it's not at all hard to stay relevant. I know many people who have been doctors, lawyers, architects, civil engineers, etc., for their entire lives and retired with some measure of distinction and the respect of their younger colleagues. Nobody ever thought to ask them why they kept doctoring, architecting and lawyering and did not go into management. Programming and systems engineering were not jumping off points to my management career: this is my profession, and fortunately it's one that doesn't require great physical agility or stamina, so as long as I am able to avoid atrophy of the brain I hope to continue at it for some years yet.
