> If your product needs to do X, Y, and Z, and that can be built in arduino in a few days, and built in some other "professional" toolchain in a few months, then you will save money by building your real product on arduino.

Not necessarily. This is only true if you only think about software. Your "real product" also consists of hardware, and if you can do X, Y, and Z on a certain chip with the Arduino environment, you can most likely do exactly the same on a smaller and cheaper chip without the Arduino environment. Simply replacing Arduino C++ by plain C code without useless abstractions can already give you a very noticeable performance improvement and code size reduction on small microcontrollers. Development time is essentially a one time investment, while the chip price is a fixed cost for every unit you build. It's purely a question of how many units you want to build in the end. And for the record, if you can build it in an Arduino environment within a few days, you can definitely build it in "some other 'professional' toolchain" in just a few more days at most.