Cybersec is so insanely broad you could do a different job every five years for the rest of your career. Or one job for 20 years like me, despite being easily bored, because every new project is different and there's always new technologies to learn. And you probably have job security for a while yet especially if you are good. For most roles, I doubt AI will replace a decent cybersec person for several years, though it may be a force multiplier.

I haven't done sysadmin in a long time and the field has had more than a few major paradigm shifts: from bare metal to virtual, virtual to container, devops, software defined everything and host as cattle not pets.

Back in the day it was a mix of projects and schedules and emergencies. In other words every day was different and a bit unpredictable. It may be more boring with modern approaches and technologies to significantly improve uptime.