What is being said here flies in the face of everything that has been learned within the windsurfing community for the last 30 years.
When Dacron was used, the rig was easy to overpower and had a very narrow wind range…the Dacron/Mylar laminate was better…then battens were added which helped increase the range, then camber inducers to force more shape into the sail. All these things helped, but when the sail manufactures started to use Mylar film and scrim/Mylar sandwich things really started to happen…all the sudden one sail could cover the wind range of 2-4 Dacron old school sails.
The designer could then design the shape into the sail and be sure it would keep its shape in a wide wind range, if set on the correct mast and used with low flex carbon booms…
The sails became much more responsive to tuning…if you set the down haul or out haul a certain place, it stayed there until you changed it…the wild fluctuation of center of effort in the sail was now gone…as the wind picked up by proper adjustment of the down haul and out haul the center of effort could now be brought down lower and moved forward on the sail making it more controllable, with the head bleeding off excess power that would slam a sailor with similar size old school sail under equal conditions.

You can’t keep the design foil in as wide of wind conditions if you have a fabric that is constantly changing shape. Dacron is generally cheaper, and more resilient to rough handling, although I have had just as many Dacron/Mylar windsurfing sails delaminate as I have Mylar, mylar/scrim/mylar sails.

Bob

First of all: It is illogical to suppose that a sail should "keep its form on a wide wind range". No matter what this initial form might be, it's evident that it should change as the wind increases. If directly asked the question "shall we sail in different wind conditions with the same sail shape?" most yachtsmen will (correctly) answer "no".

I think the main question is simply "what instruments do we have to change sail shape?"

Nowadays people think that the only way to alter sail shape is through (a) sheet tension, (b) halyard and outhaul tension and (c) mast bend. The sail is considered basically a passive subject and would just suffer the consequences of the changes in trim. In this sense, the more stability the sail had, the less it would impart the efficiency of the mentioned trimming resources.

This idea is not illogical, though I find it far from luminous. It is, nonetheless, very widespread. Why is it so popular? You have pointed to some of these reasons, which include commercial interests and profit. But there are other reasons of a more human nature that may account for the popularity of the concept:

(a) Boats became quite fast in the last decades. Fine tuning of the rig and of the sails are a time consuming task. Many yachtsmen think that tactics, for instance, are more important than yet another little pull at a downhaul or a bit more tensioning of an outhaul. Windsurfers are the quintessence of this new concept. You trim the sail once and go racing. The mast and a clever geometry of the sail will do whatever may be necessary as sail trim goes. Many fast small boats are raced in the same way.

(b) Sail shape is, in fact, something VERY elusive. It may be easy to state that a certain mainsail should have 5% camber here or 10% camber there. A totally different issue is to determine these numbers by just looking at the flying shape of a sail. Most yachtsmen simply cannot state what cambers or twists their sails show in a certain moment. A dare say the error are no less than 50% in most guesses. Thus, if a sailmaker is able to make a somewhat "rigid" sail with a preformed shape, everyone will be happy. At least one will know that the flying shape at any given moment will be not dramatically different from the initial shape. With a help from the rigging the boat will probably have sails that will change shape in accordance with the accepted paradigms (flatter sails and more twint in a breeze, for instance). For those unable to precisely evaluate sail shape, a discussion about the whole issue is like hearing to a speech in greek. Sails that boast "a predetermined shape" end up as quite wellcome.

(c) Racing results tend to confirm the idea that "rigid sails" are unsurpassed. Of course this should be so, anyway, since they represent an ample majority of the sails presently racing. Not many mathematicians are asked for an evaluation of the technical consequences of such a biased test.

In a continuous feedback system, sailmakers and yachstmen have reinforced the concept of the rigid sails. In the beginning it may be that sailmakers did not fully believe the story. They demonized flexibility to be able to convince people to buy sails made of rigid cloths like "yarn tempered Dacron". Very cumbersome, these cloths profoundly displeased people used to cotton or to soft Dacron. But little by little the better performance of these sails finished convincing everybody they were better suited for racing.

(I will add a couple of paragraphs here later)

Most yachtsmen don't know much about sailcloth. In general they simply know what they read in magazines and technical sites. Since quite often the text is accompannied by an appraisal of the authors, the readers "buy" the whole idea and are unable to discern what is just technical data and what is bias.