A point of clarification which I think you covered well but maybe isn't that clear to people observing from the outside: no one is proposing merging "incomplete" endpoints into core.

The distinction may seem semantic but it's important. A single endpoint (one for reading, creating and editing posts, for instance) may be considered "complete" on its own. It doesn't need to be accompanied by every endpoint we can think of for WordPress (eg - comments, users, site options, widgets).

An analogy that may clarify things for those that aren't yet familiar with the "endpoint" terminology is posts. Custom Post Types extended WordPress in important ways. But WordPress was not "incomplete" when it only had Posts and Pages.

Likewise, when the REST API team proposes the four primary endpoints for core, they're not suggesting a bunch of half-finished work goes into core. Even with Drew's suggested approach, the endpoints they are proposing for merge would only be merged when they're deemed complete by the core committers.

So the dispute is not over whether or not to merge something incomplete. The dispute is over whether to merge none of the completed features until every potential feature is completed, or to merge completed features as they're completed.