From a pragmatic perspective, I'm not sure this can gain traction. The mail exchange protocol was built in a very different environment, and you'd have to get some serious weight behind this to gain much traction.
To me, the more intriguing question is this: what sort of _new_ task management apps could and would get built on an interoperable platform?
One end result of the lack of interop in this space is that commercially viable task management apps trend toward being all things to all people. There is a legitimate need to satisfy a very wide variety of use cases.
If task management apps could interoperate on a common set of data, it might become feasible for much more narrowly scoped, purpose-built apps to succeed.
This is not limited to interoperability, cross domain / organization collaboration tool. It is almost like asking what sort of e-mail tools can be built on top of email protocol.
You are right about the commercially viable aspect. But if I wanted to make it commercially viable I would not have made it open.
I think something like this WILL be the `interoperate` set of data that you are talking about.