The problem with your assumption is you're "living frugally" not for the ten years to make your portfolio withdrawal rate, you're doing it _for the rest of your life_.

You live on $30K / year (in the Bay Area/NY, where you're most likely to find a starting / junior / midrange position that offers $140K - remembering that needs to be liquid)...

and after your ten years of "living frugally" (and let's be clear, it's gonna be frugal with $2500/month rent for a 1br 600 sqft apartment eating up well, 100% of your after-savings income, so you're going to be rooming with someone)...

That's what you've got for the rest of your life. Your $750K nest egg will give you an 80% chance of being able to supply $30K/year. Sure, now you can use it to work on your own stuff, but until (and unless) that catches on...