I think you're right in that the trend started with Outlook, but at least for a while Microsoft offered an opt-out in the form of Outlook Express, which was just a mail client.

My preference is that email and calendar are two entirely separate apps, because I have my calendar open for about half of the emails I compose, and the remainder of my calendar usage has nothing to do with email at all. This is one of the reasons I've stuck with Apple Mail + Calendar through the years.