jandamaschke in terms of performance, I don't see the OWA client being anywhere near as performing as Outlook, even ignoring the fact that it just works online and has perhaps 1/100 of the functionalities (and 1/1000 of the manageability, even with Office Cloud Policy Service). Zero extensibility (doesn't even works with AIP and I know a lot of big corporate customers using it).
I'm not inclined to thinking that the things that I mentioned are "Legacy", probably it's just not the technologies that the cool kids on the block are using these days, but are the ones that are keeping the world spinning.
The cloud-only dreams (both in term of operational maturity of the platform and manageability) are far far away from what the Microsoft corporate propaganda heralds. They are surely fine for simpler deployments and are a blessing for those realities who have a simple IT (SBS anyone?), but they are just not ready for the corporate, mission critical world. It has diverging requirements from a cloud deployment, you just can't give that control away or the first time Microsoft botch a patch or a service you are in big trouble (even with all the cool stuffs like KIR...).
If that wasn't true they would not be offering SharePoint Server, Exchange Server, Skype for Business Server and System Center and Configuration Manager (and I can go on) all of which have received and are still receiving tons of new functionalities with every release.
The future, for the foreseeable future, is a hybrid one. Microsoft knows that. Just sometimes they try to push too hard...