To summarize: Pre-Delrin Simplex RDs were some of the best available in their day. Campy vs Simplex was a horse race in the 1950s. In the 1960s, Simplex went big for Delrin. You could bend and twist those RDs without breaking them. That also meant that they were too flexible to shift as well as other options -- stiffness matters. This is when Campy started crushing Simplex, especially once the Nuovo Record came out (my personal nominee for the prettiest RD ever, although I have to say that the Huret pictured above is awfully pretty).

Then it turned out that the Delron RDs simply did not last. It wasn't just a matter of poor maintenance or bad adjustment - too many of them simply fell apart. Not all of them did, but waaaay more than should have. No Campy or SunTour or Shimano (or Huret, for that matter) ever just fell apart, no matter how otherwise crappy it was. Plastic Simplex did, and often shifted like dog poop before that to boot. That destroyed their reputation. And a destroyed reputation is a very hard thing to fix. Simplex simply did not have the financial resources to ride out the bad times, especially since Shimano and SunTour made more reliable and better (in the case of Shimano) or markedly better (in the case of SunTour) performing RDs at similar price points. By the 1980s, Simplex could have made an RD that was the cycling equivalent of the Second Comin g and it would have had a very hard time selling enough of them.

BTW, yes, Bernard Thevenet won two Tours de France on Simplex-equipped Peugots in 1975 and 1977. But he also had someone else maintaining them and replacing them at the first sign of any trouble, so that is not an entirely fair way to judge. It is telling that, by that time, even the French teams riding Gitanes, Le Jeunes and Merciers were all Campy-equipped.