There are some people that use Outlook for...well I'm not sure what but things that go way beyond email and calendar. I've been using the web app for several years now, it's fine. When I was new in IT, I always struggled to see what the big deal was with Outlook desktop. The web mail has folders, rules, shared mailbox support, integrated calendar, etc.
What more do you need out of email?
Well, turns out a lot. People treat email has a permanent data store. I've encountered folks with multiple PST files archiving 10+ years of email. I ran into people that needed to queue up a bunch of offline emails in their outbox to send when they're on network again (ok, I kind of get this use case), and I came across all manner of horrors of COM Add-ins.
Anyway, the root of the problem is people using email for everything it was never intended to do or be. If "new" Outlook can break some of those habits, we'll all be better off for it.