Those figures do not justify giving up a job in a cheap cost of living area. These numbers are about 60% more than corporate salaries where I live, but the cost of living in SV is roughly 3-10x greater.
Working for one of the big public tech companies pays way more than these numbers indicate. For a senior engineer RSUs should be anywhere from 50-150% of base. Base should be at least $180k if not more. Total gross for the year is in the $300-400k range.
If you live a similar lifestyle in SF as you did elsewhere you should be able to build wealth relatively easily. Assuming the economy doesn't completely crash I intend to buy a house here in the next 5 years; whether I pay all $1.5m in cash or have a mortgage depends on how the stock market does.
Your point doesn't necessarily follow from your argument. For example, if salaries where you live were $1,000,000 per year and the cost of living was $10,000, you would gain $600,000 per year in salary and lose up to $90,000 per year in costs, for a net profit of $510,000.
Having said that I think you're right. Also, IMO one should not compare the median engineer in SV to the median engineer elsewhere. A median SV engineer moving to LCOL should be able to do a lot better than median LCOL comp.
I would expect Apple and Google to pay significantly more.
But, to your point, even at Apple or Google salaries, moving to SV would be tough for somebody mid-career (ie, with a house elsewhere that likely doesn't have enough equity to swap for a house in SV).