I was lucky to be an old sack. I moved from mainframes to so called open systems, around the time when IBM decided to ship OS without source code. And when the dot.com burst, I had my mainframe skills, still rusty but good enough that my Y2K PTFs had been the ones distributed by CBT Tapes. So I was in hot demand to supervise Y2K tests, and later to migrate mainframe scripts and applications to Unix.

I did not stay in enterprise for long. I like mainframes, but I hate the culture of big companies. I next helped founding a university, and when tired by ivory tower, I left uni to fund a company to focus on machine learning for industrial applications.

So, most important advise from an old sack: stay flexible, and dont be surprised that old skills in solid technology might be worth good money at the moment when the hypes end.