Hopefully Apple will enable multiple identities for apps as part of their new push for single signon. They recently added multiple device users for schools. Enterprise users can have per-app VPNs.
Ironically, because Google and Microsoft collaboration apps don't have iOS platform control, they are much more flexible in supporting multiple user identities.
There are many scenarios where information needs to be separated (projects, consultants, cross-org teams, OSS projects) and multiple identities are a proven means of separation and compliance. This needs to be possible without Enterprise MDM.