I see no valid reasons for it to be a single app.
David Allen's vision was to create an "app agnostic meta app" that would serve as a dashboard into which you could add things in a fairly automatic way, and would integrate inputs from disparate sources (email, social media, etc.) and could learn (I presume using some form of machine learning) to manage that information to suit the individual user. Here's a good article with some more background:
https://www.fastcompany.com/3003010...pp-will-free-your-mind-and-empty-your-inboxes
Both attempts to create this technology ran into the same obstacle I mentioned above: too expensive to create, and not a large enough market.
I'm sure if some people from this forum put their heads together they might be able to create some tools that could help integrate a few software packages together. It might be helpful to some people. I wish you success with it if you choose to undertake the project.
But I think your very idea of coming up with something to act as "glue" for "best of breed" apps underscores the very challenge to creating the meta app envisioned by DA: who decides what's "best of breed"? You hate Microsoft. I don't. One of the reasons I appreciate GTD is that it doesn't force me to buy a specific brand of computer or a particular brand of software -- or even force me to use software at all if I want to manage something on paper.
DA envisioned a "meta app" that would work with... pretty much everything. That's a big undertaking.
The GTD meta app is a wonderful idea. Even though it didn't get off the ground, it was worth trying. And it may see fruition someday. It would be great if it did. But if it doesn't happen in my lifetime... I still have GTD behaviors and habits to help me overcome the limitations of today's technology.
As someone who is not a programmer, my big takeaway from these drawing was a glimpse into how the guy who pioneered GTD sees the methodology working. It gave me some ideas to improve my GTD game. Using the same old tools. At least for now.