Pretty good endeavor that you essentially created a BPM application, that is only as if email used as a BPM platform which is seen prevailing in corporate than other enterprise tools (SAP, Oracle etc.)

I actually propose instead of re-inventing email, create addons to extend what email message can be used/read/interacted with. For example, high school teaching today has been reliant on gmail for offline homework assignment and discuss between teacher and kids. There are some tools in that space aggressively exploited by schools to conduct their "innovative" teaching efforts.

Gmail, or microsoft outlook would be the best go-to destination for the majority of who can afford an in-house IT team, or shadow IT in large enterprise who's tired of the lack of evolution from SAP, IBM etc.

Still, Gmail and outlook are slow in adopting or providing email as an open extendable platform for BPM application purpose.

Then there are everyday use of email other than BPM, notification, online purchase receipt, offline discussion, even transmitting files, photo etc. via email are somewhat common with email client.

The longevity of email is exactly the lack of rigor of the intended use of it, BPM or not, email is flexible to conduct any discussion to an open goal, or without a goal.

To understand email better, one has to compare email to SMS, and social media along with how mass used these tools.