The turnaround of restaurants and bars here is amazingly fast. When one fails, another one springs up. The same street will have very different storefronts each year. I once asked a restaurant owner, on how is this possible. He told me: Rent is expensive, and if you open a restaurant that is not doing well (attracting the clientele to afford staying afloat), is it much better to shut it down otherwise it becomes a huge money pit. What often happens is this: One person signs a long term lease for some kind of business (retail, food, whatever). The business is not doing well, he decided to shut it down and sub-lease the space. (this is easier when the market rates have risen). The second guy does give a try to his dream bar/restaurant/retail. If it doesn't work, he is forced to shut it down as well, rinse and repeat until you have a business that has the right winning formula and sticks around. This is 'creative destruction' on steroids.
What I have seen in manhattan since I moved here in 2013: More hipster style coffee shops. More pizza places. Less "cheap" retail, and more luxury good stores.
So, he is right, that cheap retail is generally pushed out because of the amazon effect, but in the other hand things like "thrift / second hand clothes" stores, are doing well.
What is replacing those 'cheap' stores is generally things in the food business: Cupcake stores, Ice Scream Parlors, Yogurt bars, and yes nail saloons as well.
But my success story (at least for my part), are coffee places. Birch Coffee, Tobby's Estate, Greggory's Coffee, etc.... all started in NYC, and they are striving and opening in countless locations.