> In practice 'good iteration velocity' is often dependent on using a language/stack that you and/or your team is comfortable with and proficient in.

This is true, but some stacks impose such a high cost that the costs exceed gains from familiarity. Basically you need to evaluate the savings of switching to a better stack and how long it's going to take before that savings equals the cost of switching to the new, less-familiar stack. For greenfield projects, the cost of switching is just the opportunity cost of slower initial development velocity while the team learns the new stack; for existing projects, there may also be the cost of rewriting in the new stack, which quickly becomes prohibitive.