Interesting, but simultaneously while in 1910 human density may have been at its highest with tenement housing and inhumane factory conditions, Manhattan is certainly at its greatest "building" density at this point. Since 1910, the Manhattan economy has drastically changed and low-rise factories with up to 50+ people per 1,000 square feet have given way to class A office towers containing anywhere from 10 people per 1,000 square feet in some South Midtown tech-oriented buildings to as few as 1 person per 1,000 square feet for prestigious Park Ave towers. Large working class families of 6+ living in 2-3 rooms have given way to single young professionals living in more spacious (by yesteryear's standards) studio/2 BR apartments and wealthy families taking up to and beyond 1,000 square feet per member of household.
Also, in 1910 "tourism" was not even a tangible industry in Manhattan. Hotels were oriented towards housing people. That style of hotel is relegated only to parts of Harlem or the Bowery at this point, and frankly the only city with this widespread hotel use left is San Francisco, where they are known as SROs. Today, there are hundreds of thousands of hotel rooms in Manhattan catering to tourists from all over the world who are there to visit, shop, and spend money. This greatly adds to the building stock of the island.
On that note, in Manhattan in 1910, retail targeted the poor worker with few stores targeting the small upper class towards Central Park. There simply wasn't that much "retail" as we think of it today (for instance the kind of retail we'd love for downtown Jax to attract wasn't even so much a thing in the Manhattan of yesteryear). Today, the retail industry is one of Manhattan's strongest/largest, with the highest rents in the world, by far, achieved in multiple districts in Manhattan with stores catering to tourists and a very wealthy retail base.
I could go on, but whereas in 1910 you had >2.3 million people crammed into low-rise tenements working in low-rise factories, today you have an island with a daytime population of 4-5 million people, tourists and office workers counted among those, and a permanent population of 1.6 million.
