Office 365 comes with a very good web verison of Outlook, which supports more modern features than most email clients do (such as Office 365 Groups).

I suspect it doesn't bother them hugely that desktop client support is falling behind on any platform.

edited to add: Oddly I can't find much marketing by Microsoft of the browser Outlook, otherwise I'd link it here, but I'd say it's about 85-90% feature complete compared with the desktop client.