I don't think you need to spend all your spare time making side projects or learning new technologies, but I do think a certain amount of professional curiosity

I don't think you need to spend all your spare time making side projects or learning new technologies, but I do think a certain amount of professional curiosity is required as a software engineer, just like any other programming job. I'll probably spend 15-30 minutes every day outside of work reading about new things, or tinkering around with something, even if it's just reading Hacker News, often these things I learn end up being used at work, so I gain new skills at work. It sounds like you're in the same boat. My father is a network engineer, and he'll spend time in the evening doing things related to his field, he has a very impressive home networking setup. My friends that are electrical engineers play around with electronics in their spare time. I think that for any professional, to be successful and have a good career you need to have at least some outside interest in your field. Obviously we all have hobbies, I don't sacrifice those because of work, but if you're working in a job where you have no interest you'll never get far.