Dave Mark points out an little known aspect of iOS 10’s ability to “uninstall” default apps:

Interestingly, some apps have a fallback and others do not. For example, if you delete the Calendar app and have Fantastical installed, Fantastical will automatically fill the calendaring role (you can add calendar events to Fantastical via Siri, for example). But if you delete the Mail or Calculator app, nothing fills the gap, even if you have alternatives installed. This is a beta, so that behavior might change.