As my 'name' suggests, I recommend maintaining competency in myriad disciplines while focusing on the 'hot' items currently in demand.

My current employer needs me to handle mainframe (IBM Z/OS), Windows servers and desktops (including apps such as Office, Outlook, etc.), Unix (AIX and Solaris), Cisco routers, Avaya phones, and anything else that comes up.

I don't have to be expert in all those, but I need to know enough to fix most things and triage the rest. I could be wrong but I seriously doubt I have to worry about job security.