It's a $1.50 MCU at low volume which probably has off the shelf USB drivers. Why would you build-cost-optimize a prototype for a product you never plan to mass

It's a $1.50 MCU at low volume which probably has off the shelf USB drivers. Why would you build-cost-optimize a prototype for a product you never plan to mass produce? Choosing the path with lowest engineering cost is almost always the correct solution in such a situation.