This is exactly right. It is a false choice to suggest that "open" means "slow". But that is fostered in part by folks who think "open" means "community driven."

It is perfectly reasonable to publish the specs of what your doing with no commitment of support, simply for folks to see and possibly use at their own risk.

That said, the reason companies don't do this is often one or two reasons. Reason one is that it results in a 'noisy' side channel as people who read the specs share their opinions, but they also get bent out of shape if something they said, or think they said, appears in the spec later. The other issue is that there is always a sanitization/prep process to take essentially internal information about a service and create an external representation for consumption (even if that consumption is unsupported).

So the 'easy' choice is to not publish.