Office 365 moves work to the clouds

... like a snow job. The effort to maintain the application is still there but now it is M$ job and not yours anymore. And I bet they don't do it for free.
And if you think a bit in the future and you do this with every application in your company you save the money for your own server room but you will need the same money to take care of your data. You need a much dearer internet connection and much more.
And who says that you don't have the same problems if the machines were yours? Will M$ migrate all your documents and macros for free to the next version of their service?
I don't see why there should be a different ROI by switching just the maintainer.

The advantage to business would be scaleability, to be able to rapidly add or reduce the number of clients and a lower IT cost - no upgrades to manage, installations. It will be a matter of the ROI of rent vs own.

... as the users are paying for enough services they will notice that they pay in sum more with the micro payments as if they had bought the boxed version. Sure there will be a few who use e.g. word once a year but I think this will be a very small group of people.
The next thing will be to offer Office through Steam! Maybe they'll get some more additional customers.