The OP here mentions that they can just go to another shop and get work if they do not like their current shop. That's fine, but the employers can do the same too. They can fire you and get another person as well. Granted, right now, the employee holds a lot more power in the market, but that will ebb over time, and it may even switch.

I don't know, on the one hand I agree that S.P.O.F. in the technical sense doesn't translate to the HR sense such that there will always be someone who can come in and learn everything needed to handle the job.

On the other hand its like a mediocre army doing a great job because of a great general. That general leaves/dies and the army falls apart or is reduced to a fraction of its former self.

Especially at smaller orgs with constrained resources, they may replace their technical lead but if the replacement flounders, and the replacement's replacement flounders, the whole org becomes FUBAR. In this sense that original technical lead becomes ipso-facto irreplaceable (IMO).