I wonder: What is the average downtime of Google Mail / calendar vs the average large corporation's exchange server? Another thought- when Google calendar / mail goes down - it's likely business users blame Google - not the IT group. When a self-hosted exchange server goes down, IT is probably blamed. Leaving an incentive to move toward cloud providers just to shift the blame.
If you or your provider succeeds, you both can claim victory.
If you try to upgrade/restructure and fail, everyone blames you. If your provider tries to upgrade/restructure and fails, you can blame them, and everyone in your company agrees with you. So politically, you can save yourself from "losing".