This isn't invariable at all. More often what you throw together gets thrown away. I'd estimate this happens to more (working) code I've written over my career than not. The hardest thing to get right is often getting the contours of a tool right and ascertaining what it should do - not making it work right after it has proven itself.
>Using a "stiff" set of tools might slow down the first stages of prototyping, but it also makes the prototype easier to modify as requirements change.
This is only really the case where the prototype was fundamentally solving the right problem in the right way to begin with and the subsequent changes are incremental in nature. If the design of, say, a microcomponent is flawed from the outset or it did the wrong thing and you have to re-do it, those "stiff" tools slow you down.
Using an extremely strict set of tools in a prototyping environment also invariably means a lot of extra up front work dealing with the tools' attempts to protect you from bugs which have an extremely low probability of occurring and/or an extremely low cost when they do occur.
If F# or rust or haskell really was quicker and more effective in a prototyping environment as well as when writing production hardened code, programmers would likely eventually converge on only using them. That isn't what is happening.