I don't even live in SF and this is clear to me. Or maybe it's clear to me because I don't live in SF?
Even just finance is probably strong enough to support SF. Or, like Vancouver, lots of people from Asia. The tolerant/hippie/gay/etc. angle might be enough as well.
SF's revitalization has more to do with that than with "tech" jobs. I know in the Hacker News echo-chamber its hard to think there's anything but Internet Tech in SF but it's got ALOT more business than that -- It's the largest banking center on the west coast, got a massive bio-tech footprint, has some major apparel companies (Levi and Gap and a bunch of smaller ones), etc, etc. It's pretty diversified.
You can see this happen in many ways aside from city's becoming safer: Real estate cost increases in cities, The whole bike-movement is an element of it, NYU getting 44k applications, the growth of the Brooklyn "brand".
I think, generally, there's a real desire for an urban community with culture and shared experiences. I know I don't have anything more 'concrete' to back this up, but this is the general sentiment of my friends (in their late 20s to early 30s.)
There are tons of tech jobs available in SF. Arguably, there are too many jobs available there which is causing at least some of the cost of living pressure. These jobs are unlikely to vanish en masse like they did in Detroit (at least not any time soon), so even if a bunch of tech people leave SF, many will stay and it'll just move the city towards a natural equilibrium.
It wasn't until the Big 3 were on the brink following the banking/liquidity crisis that the greater auto industry started shedding jobs. And by that point, the jobs weren't even in the city in the first place. The losses were suffered by the suburbs.
The job problem in Detroit was that cheap greenfield development deals drew all the new jobs into the suburbs. (with an assist from the highway system, race relations, city politics, etc) And with the jobs, went any economic reason to live in the city center and quite a huge chunk of the tax base.
Despite the difference in reasons, the challenge of San Francisco does share a somewhat-familiar shape with that of Detroit: the rich can justify living there, the young can romanticize it, the poor have little choice but to stay, but the lower and middle classes, and anyone with a family who can afford the choice makes for the exit.
Pittsburghers really believed in their city, even through the tough economic times. If anything I miss about Pittsburgh it would be that. An older generation of suburban Detroiters would simply have no second thoughts if the City of Detroit burned to the ground. That was one of the most jaw dropping things about moving here, the lackadaisical attitude towards the state of the city among the older generation.
Although IME it actually seems like the tech industry in Detroit is humming along quite nicely. The auto industry is back again and there's a resurgence of startup and entrepreneurial spirit around these parts. My company is hiring like mad.
However I feel like the local economy is struggling to find people who will actually move here. Even if there are perfectly livable areas in the region, very few people have a good mental image when you say "I'm moving to Detroit!"