> and at worst it was commercially unworkable ("Welcome changing requirements, even late in development")

It's been workable for me. We can change the requirements as late as you like, because I'm getting paid by the hour. Scaled up to a company, that translates to not giving a fixed price for anything.

If you need to give a fixed price, either be experienced enough to know by how much your agreement can change and factor that in, or turn it down. You should also turn down demands for a fixed price on novel solutions you can't have experience on.