But it's also important to note that only $300K of that is in cash. The other $600K is in profit participation, which could take years (maybe even a decade!) to be realized. It could also be worth $6M a year when it's realized.
But ultimately it's an investment of your time. Or to think of it another way, you're getting paid $900K a year but you're also investing $600K a year in OpenAI, which may end up being an amazing return or nothing at all, just like any startup investment.
Although with Sam at the helm, my guess is it will probably be worth more than $600K a year.
First, ~46% of it is gone in taxes including federal tax (~25%), state tax (~8%), FICA (~4%), and sales tax on everything you eventually use the money for (~9%).
So that's 162K left. Not a lot to pay sky-high rents, car payments, insane medical bills despite insurance, lawyers to fight said bills, save up money for parental elderly care, save up money for yourself for retirement, etc.
And yeah, having kids on that money? Very difficult.
If you're not in the bay area, different story, it's a very nice income. But they probably won't give you that package if you're remote.
And if you're in the bay and not planning on having kids, it's an okay salary.
If you can't retire at 65 on 300k a year, you're not living a middle class lifestyle. Not even close.
Rent: 4k/month = 48k. Car payments 750/month = $9k. Using your post-tax numbers (which are wrong) that leaves over $100k per year for all the other random stuff you've listed, which is more than most people in the Bay Area make in a year.
(Also, you're math is wrong on the taxes; the rates you use are the statutory progressive rates, not the effective rates (so, for example, the effective rate on $300k would be approx 22% at the federal level assuming standard deduction but no retirement contributions or child credits). FICA is capped at the first $160k of income (meaning you don't pay more if you make more).)