Until we truly solve remote, there will be some concentration. And there's value to that as an employee. If your employer goes kaput, or you're stagnating in your career, or you have an abusive boss, there are plenty of opportunities available, and enough of them that one shock to the system won't break the local economy.
Of course, the flip side of that is that real estate values go through the roof.
Why would you go to the other extreme? There are hundreds of large cities around the US that could easily headquarter these big companies and not become company towns.
How small can a city be and have a healthy relationship with an employer of 25,000?
A company like Amazon will have some natural needs. No other city can really beat NYC for meeting those needs.
Not Chicago. Not Toronto. Not Raleigh. Not Jersey.
And certainly not Texas.
By air, land, and sea the best and most bountiful transport links to the rest of the US, or to the Europe-Africa side of the world, are out of NYC. Now maybe we can change that? But we don't seem to be working towards changing it. So as of today, and for the foreseeable future, NYC is the best hub.