never have i ever been involved with a _successful_ software project where product decisions were made entirely by product people and handed off to design/eng.

in my limited experience, good product builders exploit leverage points that in order to solve customer problems / test assumptions quickly. for example, usually the product people don't know how flow X is really easy to restructure like Y and solve the same problem. kind of related, but pushing all the speccing responsibility to product is also kind of boring for me as an IC.