The reality is that Kodak was working on substituted PPD's and many other chemicals of that sort throughout this time, mainly for colour films - indeed, Photo Engineer (aka Ron Mowrey) who was one of the peer reviewers of Haist's book, is named in the relevant patent as one of the inventors of CD-6 as used in K-14. That Kodak chose not to use these developing agents in BW developers suggests that they felt there was no good reason to do so - likely because they increasingly knew how developers & iodide placement in the grain structures could be made to interact & thus how to build an emulsion that could maximise sharpness relative to graininess. Haist is an extremely interesting read, but it is also interesting what he left out (commercial sensitivity I imagine) about the subsequent grain growth & sensitising technologies that were deep in R&D at the time he was writing.
As to HEAP of Promicrol & Atomal fame (I assume that's what you're referring to), I think they were trying to find ways to get the same speed boosting effect as Ilford's new PQ developers without paying Ilford for phenidone. In the long term all the big players went down the phenidone (and its evolutions) route for various reasons, most likely the speed boosting & low toxicity.
Don't forget that in the pre-1960 film speed world, 777 & Microdol were not seen as speed losing developers, D-76 was recommended for pushing & the new developers with phenidone or HEAP seen as sending film speeds through the roof without giving impossibly dense highlights from excessive pushing.