With the release of Exchange 2010 SP1, we introduced the /hosting mode switch – a feature which deploys Exchange using an Active Directory structure that affords complete separation between tenant ...
Updated Jul 01, 2019
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@ Thomas - rough timeline is before the end of the year. As to whether you should stop or carry on, while I can't tell you what to do, I would suggest you look at what you can offer with a /hosting mode deployment, look at what an Enterprise deployment potentially offers you, and make your decision on that. If there are things in Enterprise you would like to have the ability to use that you know don't work in /Hosting (such as UM) then wait for Enterprise. Not ALL Enterprise features are going to work when configured for multi-tenancy, but a lot will, and a lot that don't exist in /Hosting.
@ Martijn - You are right that using Enterprise configured to make it work in a multi-tenant way is not the same as legal separation, though before deciding whether it will or won't meet 'legal separation' make sure you, and the people who will decide whether it is, or is not, have agreed on what the defintion is. Just advice I've learned the hard way over the years with the well-used phrase 'two factor auth'. If you truly need legal separation then you need multiple forests, multiple admins, as well as a ton of process and control.
As for O365, it uses a directory layout for Exchange which is basically the same as that in /hosting and has the same concept of tenants and separation, but there's a lot more to O365 than that. It's quite different from /hosting.