"Doing one thing well" sounds great on paper until you realize what that really means in this context...
Either you'll have multiple address books/sets of contacts (you'll need one for each different application, one for email, one for calendaring, one for collaboration,) or you integrate that function between them all. Both are horrendous pains in the ass, unless the servers are written to work together in the first place. ...But if they are, you're suddenly "locked in" to using only products that are all compatible with each other. Of course, if any part of that integration breaks (likely, if the applications involved weren't written to work together in the first place, and you had to "improvise" a solution to make that happen) all the apps lose their contacts function at once. So we're back to multiple sets of contacts, and "one-tiny-failure" breaking all these functions.
And you'd have to have an account on each of these separate servers, permissions on all of them... Somebody would have to know how all five platforms work (instead of having just one Exchange admin,) and have monitoring capabilities configured correctly. That's another thing: Does XYZ Calendar App support your monitoring tool? Your backup platform? If not, better be prepared for some pissed off co-workers when it crashes overnight and you don't know that's happened until you arrive at the office the next day. Or better be prepared to find another job if your backup solution's support for it is limited, and you restore a "backup" that brings back the functionality while losing all the data.
I mean, if you really think that is "simple" then more power to you... I mean, it is your career, after all... But your co-workers and bosses will laugh you out of your job if you seriously propose this "model" which would turn the entire business' concept of productivity on its ear, since they almost certainly currently use Exchange and Outlook for productivity apps, and every single user would have to re-learn everything they currently know to make your "simple" solution work. My guess? You'd be fired before lunch the first day this "solution" went live, as angry people from the parts of your company that generate profit as opposed to just spending money on personal technology wish lists line up outside your boss' office to decry your insane, not-compatible-with-how-we-do-business decisions.
Not to mention the lynching you'd get for breaking everybody's Android/iToys integration to their work productivity apps, since those functions on both platforms rely on ActiveSync, which isn't available anywhere but Microsoft platforms.