If they're all downstream users and rebuilding their entire infrastructure on Clang might not be the most enticing thing if they're deeply embedded into EDG in their products and EDG can be kept alive with small costs per company.

I think that's the point of the whole point of a nonprofit stewardship, that a few developers each from the main customers can come together and cooperate well enough on the upstream that they don't have to take the entire cost of changing downstream to Clang or maintaining a frontend.