Nah that's not really it. The reason NVIDIA doesn't allow this is precisely because the additional RAM - functionally the only difference - is not cost efficien

Nah that's not really it. The reason NVIDIA doesn't allow this is precisely because the additional RAM - functionally the only difference - is not cost efficient. People would like (and did) use a bunch of consumer 1080s, which is why NVIDIA disallowed precisely that. You had to buy the equivalent pro grade card, which costs easily two or three times that and offers a couple more GB of RAM.