Interestingly enough, similar models seemed to be more common back in the dark ages of computing before terms like Open Source and Free Software were really a thing, particularly on minicomputers and mainframes. It made its way to the "microcomputer" world in more limited fashion, then sort of slowly died out. But I think there may still be something to the notion of a license that, while not remotely meeting the definition of "free software," still gives you access to the source.

I'll be honest, though; I think FOSS advocates have a tendency to (perhaps unconsciously) downplay how important "free as in beer" has been for the adoption of a lot of major software projects. An internet in which Apache, MySQL, PHP, and other major free software projects were released with commercial "source included" licenses would probably look very different.

I have my own thoughts about RethinkDB's failure as a commercial entity, but despite having actually been inside, I have pretty limited insider knowledge. Perhaps we should have been trying to push an "enterprise edition" with guaranteed support much earlier, following the "charging for OSS stuff" line you mentioned; as others have said, maybe we just didn't have enough avenues through which to give us money. (It's hard not to notice that MariaDB's .com site is essentially page after page of ways to give them money, for instance.)