With all due respect, except for a very brief period before Bill Shaw came on board, Pearson was not the most successful production boat builder of that era.
This also baffles me. One of Bill Shaw's greatest strengths was that he was able to produce designs in a wide range of venaculars.
My first point was merely that Bill Shaw was a preeminent and successful yacht designer in his time, just as Bob Perry, William Lapworth, Phil Rhodes, Bill Lee, Rod Johnstone, and a number of other are. I put the heyday of sailing later than you do, in the 70s, not the 60s.
My second point comes from more of an artistic and spiritual view than a technical design view. All of us have a calling and a purpose in life. If you look at his Bill Shaw's work, you can see an evolution of his design to a point where he realized his calling, his purpose in his field, the distinctive contributions he made.
To draw an analogy, in the art of writing, you can recognize a point where a particular author realizes his distinctive voice, his unique style that develops at a certain point after years of arduous technical work. From a spiritual view, that is where the author realizes his purpose, his calling in life. The same can be said of painters, musicians, and any other field where technique and art intersect. Commercial success does not necessarily coincide with, or diminish, the realization of that style.
Obviously, Bill Shaw favored certain design elements and those design elements were based on his view of how a sailboat should sail and function. Obviously, he was constrained by the science and technology of the time, the market, the business of selling boats, the need to run a company, and the desire to design to certain rules of the day.
I believe Bill Shaw realized his purpose with his design elements in the early 70s, what you refer to as his early IOR work. Several of his most successful designs came out in this period. If you were to pick a prototypical and enduring boat design, if would be the P30, followed by the P26 or the 10M. Bill Shaw realized his vision in the creation of a racer/coastal cruiser for a small family with those combination of elements.
You could say Bob Perry realized his purpose with the Valiant 40, or Rod Johnstone with the J/24, or Bill Lapworth with the Cal 40, or Bill Lee with Merlin or the Santa Cruz 27. Of course, all designers and artists further refine their style or work over the years, and one can argue that they created better work or greater success after that point, but there is a point where they make their most important contribution to their calling. You can also see where particular designers incorporate the work of others into their product. For example, can anyone deny that the Pearson Flyer shows an influence from Rod Johnstone's successful realization of his particular vision in the J/24?
I am not saying all Bill-Shaw-designed boats sail alike, but there is a distinctive feel to the realization of his design in sailboats, one that I favor. If you read the History of Pearson Yachts article in Good Old Boat, it is clear that Pearson succumbed to market forces in the '80s, first by attempting to meet the demand for condo boats, and then, by the recession. IMHO, Pearson Yachts peaked in the 70s, as did Bill Shaw.