> developing isn't for developments sake.
[citation needed]
> the person who makes that product reaps all the monetary benefits from the work that the others have made
Which means that they can offer their product for a lower price, which then benefits the general public.
Companies being able to operate cheaper / more efficiently does benefit the general public, as long as the market isn't a monopoly. And as per my above comment, most markets are not monopolies.
> open source developers never make a product polished and user-friendly enough to be usable by the general public
I've been using Audacity, Gimp, Inkscape, uBlock Origin, and many others long before I knew what FOSS means. Spliit is also pretty cool ;)