This. Unless a written guarantee could be offered by the cloud provider that migration will be absolutely trouble free, why wouldn't customers just stick to what they know works?
And if they're not willing to provide a written guarantee, then that says a lot.
In this scenario, it's nothing to do with the cloud provider. Migrating a live production system is inherently difficult. You can make a reasonable analogy to moving houses – say, with two kids who are in school, and while you and your spouse are both working. No matter what guarantees you're given regarding the condition of the new house, simply moving all of your stuff (while you are using it) is a big hassle.
In this example, it wouldn't be the guarantees for the new house, which presumably would have been examined and accepted well beforehand, it would be guarantees for the moving process itself.
And if they're not willing to provide a written guarantee, then that says a lot.