My employer has had an open req for someone with my skill set for more than a year. We've interviewed a few people but even people who looked good on paper obviously didn't have the skills and experience when we brought them in. We're still looking.
There are other people out there but they are rare, highly compensated, and not typically actively looking for new opportunities. You have to find them and try to poach them.
My group is executing on or bidding on 1-2 billion dollars in contacts that hinge on my expertise. We are going to have to expand. I can't do everything that will need to be done over the next 5 years by myself.
All of this to say, yes, I'm replaceable. Just not easily. And if I don't do my annual goals, my manager doesn't care at all. We've talked about it explicitly. I'm doing what they need and I'm doing it well. The best thing my manager can do is run interference to keep my path clear to focus on what I do. That often means having other people write the reports or create the presentations that multiple people can do and let me concentrate on the work only I can do. And if I don't want to participate in the big corporate goal stuff, I don't have to. It doesn't negatively impact me on my annual raises.