Yeah, I think the version of Outlook I'm referring to is part of Office, as Classic Outlook has a toggle to "Try New Outlook," and doing so will instantly launch New Outlook and make it your default email client. If you don't have Classic Outlook pinned somewhere, it disappears into the background, but it still is accessible. I guess MS knew the new app was going to be a mess to start with, so they left you "Opt in" to start, and you can still Opt Out. I suspect eventually CO will get taken behind the barn, as it's also a mess of an app (due to its long lifespan and feature additions over time) that I'm sure MS doesn't want to keep cobbling it together.