2) startups are always growing or shrinking. no point putting so much significant capital unless it is an established in for a long haul at a location where talent is surplus.
3) if you indeed have that much capital, you can pretty much dictate what you want and get some tax benefits (amazon, second HQ) or work with a city build a new building suitable for your long-term needs (apple).
4) not much inventory that meets your needs in SF market because the property prices are always going up.
5) the leasing company actually has limited leverage. finding another tenant esp. the size of dropbox is not easy. you can bin pack other tenants but you lose lot of money until all new tenants move in.