But, I think, for a lot of people, a lot of their spending is nonlocal (e.g. internet shopping) or at least on price-standardized goods, and so is not tied to cost-of-living.
Let's say that in SV, you'd be making 400k, and—after spending on cost-of-living—keeping 10% as "pocket money." Well, that's 40k.
Now let's say that in Austin, you'd be making 100k, and keeping 30% as pocket money. That's 30k.
40k buys more iPhones (or whatever else you might want) than 30k. 40k is also more money to send back home to your family in India or wherever, than 30k is. It's smaller in relative terms, but what does that matter when it's still larger in absolute terms?
(Certainly, there's a lifestyle where spending on priced-to-cost-of-living goods can expand to consume all available salary. For example, if your goal in life is to throw extravagant parties with delicious food, then that food is going to cost less in Austin, so–even with 10k less in the budget—you can afford to throw more parties, and invite more people. But I feel like HN types are the least likely to have this kind of spending profile.)