Even if your specialization continues to be valuable, it's still something that can easily and insidiously turn from a moat to a pidgeon-hole to a set of golden handcuffs. I think the distinction comes in how portable your specialization is, ie. "I'm really good at rapid prototyping" vs. your example of "I know everything there is to know about FAX".
If anything, I'd be cautious of building a personal moat based on knowledge, because it's too easy for that to become a trap where you can't be replaced but you also have nowhere to grow.
If anything, I'd be cautious of building a personal moat based on knowledge, because it's too easy for that to become a trap where you can't be replaced but you also have nowhere to grow.