I believe, if you can maintain a genuine interest in certain, contemporary topics than you will probably have an easier time to stay relevant.
Today, depending on your interest, I'd probably would go deep into the "devops" field, since that's quite technical, fast moving but with lots of stable parts (like operating systems) and there's demand and will be for some more time.
There's also always a way to stay relevant by just being very good at a very particular thing - even if there are only a few jobs. I remember mainframe people advertising that if you know your way around z/OS you'll never have to search for a job again.
Today, depending on your interest, I'd probably would go deep into the "devops" field, since that's quite technical, fast moving but with lots of stable parts (like operating systems) and there's demand and will be for some more time.
There's also always a way to stay relevant by just being very good at a very particular thing - even if there are only a few jobs. I remember mainframe people advertising that if you know your way around z/OS you'll never have to search for a job again.