> What large corporation would ever sign up to that?

Ones with the power to have some very nice contract terms put into place to mitigate downsides in the medium term. If it's a problem long term then that'll be the next CEOs problem and the current one will have cashed in their $50m check already.

Less cynically, for many of them it's either being captive to their barely working IT department or being captive to a decently more competent AWS. So they choose the one that will give them better features for less money in the medium term. They can't make their IT work better because it's a two sided monopoly. Only one client who is guaranteed to not leave (until AWS at least). That inherently and unavoidably creates horrible incentives.