cloud switch is more complex?
"But let’s all acknowledge that the cloud switch is more complex than some would lead us to believe"
Actually sounds pretty simple if they managed to migrate thousands of users from Exchange to a totally new system with only 1 day or less outage for some number of users (users will whine about everybody having no mail when its just them and their mate from the next desk). I have seen Exchange servers down for longer just with an upgrade or switching spam filtering providers.
I am guessing the IT dept thought it was simple as well; but underestimated what sheer volume can do. Probably ran a trial with 50 users, all went well so though 10,000 accounts would be no different.
Only fail here is not migrating users office by office or dept by dept to deal with training or any scaling issues. Sounds like migrating a few hundred or few thousand users per week would have been a more conservative approach. They must have considered the effect of just transferring mailboxes, each hundreds of MB for thousands of users, would have killed a companies bandwidth.