Both in regards to decreasing the cost of living and also increasing prosperity for everyone, the clearest answer to me is to purposefully and meaningfully increase investment in areas outside of San Francisco.
The Valley, through a variety of circumstances, is now home to the highest concentration of technology companies, and is unsurprisingly one of the most expensive places to live in the world.
The are known economic forces [1] that cause clustering of similar firms in small areas, and there's no doubt that this is a net positive for both the city and the individual businesses, at least at first. The incredible concentration both of talent and wealth in San Francisco and Silicon Valley is a clear testament to the power of physical proximity.
However, I urge the technology community and specifically Y Combinator to seriously reconsider the notion that all of technology must happen on a peninsula in central California.
How this decreases the cost of living is obvious. I live in Memphis TN and I pay $500 a month on my mortgage. Further, the city of Memphis and many other cities in the US are in serious need of both talented young people and economic development. The "brain drain" is a real and devastating effect of compressing the technology ecosystem onto the West Coast. Reversing the brain drain would bring leadership, talent, and dollars to areas that need it.
I'd like to specifically call out YC for this section of their FAQ:
> Can we do it without moving to where you are?
> Sorry, no. We tried this once, and by Demo Day that startup was way behind the rest.
In every other aspect of their model, YC is open to all reasonable possibilities. They fund photo sharing apps and nuclear reactors, women and men, young and old, but NOT people who won't move to SF. This reasoning is apparently based on just a few examples. Thus, the idea that constant physical availability is a fundamentally important part funding is just conjecture.
This policy and the attitude behind it are essentially opposed to the ethos of the technologist. How does one change the world by adding incrementally to the wealth of the Bay Area? Also, it's an absurdity that I have to mention that the internet was invented so that we wouldn't have to do stuff like this anymore.
This also addresses the issue of diversity. If you want more diverse people, go to where they live. I find it an amusing contradiction that many of the people who cringe at the idea of moving to the repressive "Deep South" of Tennessee would find themselves working with people who aren't white dudes for the the first time if they ever did move.
So, I urge YC and the tech community at large to spread out from just the bay area. Reconsider the cities you came from. Be bold and try to find the opportunities that are waiting outside Silicon Valley.
[1] https://en.wikipedia.org/wiki/Business_cluster