"If you can't replace someone, fire them."
"If you can't replace them, you can't promote them."
Some of the things people say about job security are, quite frankly, bizarre. Take that first quotation: what exactly is this supposed to accomplish? Is this Chaos Monkey, Bad Management Edition? And the second one: you're secure and you're also stuck.
Being Irreplaceable Not Necessarily a Good Thing
I've been irreplaceable before and it seems to be analogous to "siloed." Sure, I had fantastic "job security." I also at the same time had no sanity, no vacation without constant interruptions, and no help. Maybe the job was secure but my chances of intentionally staying in it, not so much. I think when someone becomes irreplaceable this indicates a management failure. You must spread the knowledge because it's either tribal or (as we say around my house), out of date as soon as you write it down.
You should strive for the security that comes from being good at what you do and being on a good team (or be a consultant!). I watched as a company lay off the one person who knew how to manage its internal ticketing system and then observed with unsurprised horror the aftermath a mere week later when it broke.
Want to Succeed? Cross-Train
If you want to succeed, you must cross-train. If you want your business to succeed, you must not deliberately isolate employees or allow them to hoard knowledge; they're hurting themselves and may not even know it.
You wouldn't use RAID 0 on a mission critical server, would you?