Microsoft decided to integrate Exchange to Active Directory a long ago which I think was a bad decision from cloud era point of view. Everything works fine in one AD Forest but when you need something else it can became a really nightmare. Basically Microsoft is trying force to change the system (which was originally designed only for on-premises environment) to work in the cloud with variable success. We all know how tight Exchange is integrated with AD (especially Configuration partition). In the past it may seemed a good idea to store all the configuration information in AD but it has become a burden. This O365 mess is just too complicated to handle for Microsoft itself. Google and the other cloud providers didn't had that background and they could start from the scratch. To separate Exchange from AD could be one solution but backward compatibility might be loosed but that could be a wise decision in the long run. It would be a really nice to run several Exchange instances in your on-promises network and in one AD! Now you have only one Exchange organisation in AD/Forest and every time you want to install new versions from Exchange you have to upgrade that one Org and keep fingers crossed that nothing breaks. It looks like version 15 is some kind of "Exchange Vista" something that we can forget as soon as possible. Looking forward to see where the Exchange will be going… I still have high hopes.