I recall reading about this a while back, could hardly believe they could be that stupid.
"Real" Pyrex (borosilicate, as you mention) is a superb product. Glass that can handle sudden changes in heat very well.
So they built up a brand name for many decades - "Pyrex" meant quality glass to generations, and they decide to use a substitute? Geez, if you are going to do that, sell it under a different brand name then, don't compromise the name "Pyrex".
I never went to business school, but I seem to be aware that "building a brand" is a very expensive process. Why throw that away? Stupid!
Now I need to look for the words "borosilicate" when I shop for glass.
https://en.wikipedia.org/wiki/Borosilicate_glass
-ERD50