It does seem unlikely to that something like this would gain traction among existing task-oriented apps and services, but perhaps that's not the point.

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.