Even if you spend a bit and spend another 3K per month on stuff like food, bills, and some spending money, that's still almost 9K that you can save per month. Sounds VERY lavish to me. So lavish that in 15 years of work you could save up 1.6 million, buy a cheap house somewhere (or already have one from buying it while working there) for $500K and have 1.1 million to live frugally on and just work on side projects for a little extra cash.
On a 3:1 debt to income ratio, I'd put median income for the area for those who can afford to buy a home there is approx $350K (80% financing on median home price).
The fact that some people cannot live alone on that salary in SF does not mean most people cannot. I make considerably less than that and I could make it work. I wouldn't, because that would destroy my ability to save anything, but I could do it.