The area has been built up with skyscrapers over the past decade, well before HQ2 had even been announced. Development in New York is not so quick that you could shovel a project through in the four months after the announcement.
New York's economy is plenty diverse. There's real estate and Wall Street, sure, but there's also legal services, advertising, media, health, education (NYC has the largest student population in the country), hospitality, tourism, etc. This is no one company town like Gary.
I wouldn't describe LIC (or any part of Queens) as being "built up with skyscrapers" personally. I'd say there are a handful of tall apartment buildings.
New York's economy is plenty diverse. There's real estate and Wall Street, sure, but there's also legal services, advertising, media, health, education (NYC has the largest student population in the country), hospitality, tourism, etc. This is no one company town like Gary.