So where can we buy quality oven-safe glassware now in the US?
This is not a coincidental phenomenon, it's a deliberate end goal. Private equity is part of the problem, but more broadly the problem is that of consolidation and market incentives for behavior other than selling the best product at the best price. It's easy to blame government for not creating the right incentives or failing to create the right disincentives, but realistically corporations themselves have tremendous incentive to co-opt, subvert, and otherwise manipulate government decision-making processes and regulatory agencies.
These companies are just a subtler version of that.
I’m unsure if all their glass cookware is borosilicate but they clearly marked it as such when I last looked.