> Have you ever seen a payer demand a change from a proprietary vendor and get it?

This happens constantly, but in private. Usually the conversation goes like this:

    customer: opens support ticket, asking how to do some task X
    vendor: sorry, that needs a feature added to the product
    customer: fine, we'll switch to $other_vendor
    vendor: oh look at that, our engineering department scheduled the feature for next release.

Granted though that when dealing with vendors as large as Microsoft that's not going to happen.