If I'm not very mistaken, the main difference of the two approaches is in marketing, isn't it?

WP API can be marketed as WordPress grown modern (or whatever term you prefer) to developers and decision makers even outside the community, but can it when it's not "complete" yet? Complete in the sense you can do literally everything with it? Or in other words, would "outsiders" buy into the concept of complete endpoints instead of a complete API and consider the overall feature mature enough to work with?
I know many agencies and developers do already, but those seem to be mainly the ones who've been following its development anyhow.

On the other hand, trying to market a feature that hasn't made it into core yet, is not marketing WordPress, it's marketing a potential. It's just not there yet, from an outsider's point of view. Parts of it are in core, the rest is a darn plugin. Plugins break. Contributors can get sick. BDFLs can change their minds. Does this sound compelling from a marketing point of view?

If I'm understanding Matt correctly, he wants to be able to take the whole cow to the market, not a couple of hoofs with a promise tagged on them. From the point of view of a project lead, I think that wish deserves consideration beyond arguments regarding development and the benefits of developers.

Though I'm generally all for progressive enhancement, I'm not taking one or the other side here, just trying to highlight certain aspects of the problem that I think may be relevant for better understanding.