[RJVB]

You're probably right. Back in the day when I made the above mentioned "stealth" stereo mic (with the help of an electronics guy in my lab!) I had been looking at ordering a similar one made by a small company. That same electronics guy told me I'd be wasting my money, that we could make something essentially identical by just buying the components, because "everyone uses the same [button] mic elements anyway". You pay for having them matched, you pay for the nicely packaged electronics and for the housing, and the manufacturer takes a margin off all that. I already knew that argument from an uncle who worked for Philips in their German TV division: add a 10 cent component and that will increase the MSRP with at least 10 marks.
Indeed, I payed an incomparably lower price for the handful of components, in the end (the most expensive being the quality cables and the little plastic box, IIRC).

So yeah, it's probably true that it'll be easy to replace an electronics circuit inside a nice packaging with a quality transducer element (the actual mic). Much of those electronics might even be superfluous (and thus get in the way) if you're connecting the mic to quality studio equiment(?)