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Here's a speculation of what could have been if major top 10 cities didn't lose people.....

*I know family sizes were bigger historically. For this exercise, lets pretend people moved out crowded households and into new infill and high rises, keeping the numbers the same.

Cleveland, Pittsburgh, and Baltimore would all be around 12,000 ppsm. Cleveland would probably be arranged like a smaller Toronto today (strong downtown with scattered nodes of high density), whereas Baltimore and Pittsburgh might resemble Montreal, (or a hilly Montreal)

Detroit, Buffalo, DC and St Louis would be around the 13-14K ppsm range. Detroit would also be similar to Toronto (Woodward akin to Yonge with New Center similar to Yonge/Bloor) Buffalo would be smaller, but similar. Ironically with St Louis, I see it looking more like DC does today (as jpdivola also stated), with very elegant, urban but spacious residential streets. DC itself is very close to its peak, so it's hard to see where it might have looked different. Perhaps SE DC would be more vibrant than it is today.

In the 15,000 - 16,000 ppsm range would be Philadelphia, Chicago and Boston. Boston, like DC, is also somewhat close to its peak population. Chicago and Philadelphia, however, would be fundamentally different. Vibrant areas would be spread more evenly across the city.

Two anomalies are New Orleans and Cincinnati, which both have clusters of very dense significant (wealthy) urban neighborhoods, but historically also have very large boundaries, so the density figures at their peak aren't that high. In Cincinnati's case, it also has walkable urbanity that extends across the river into another state. The closest comparison I could come up with may be a Vancouver - A large, contiguous central area with an extreme jump in density from its outlying areas. New Orleans leaning more towards SF, and the Natti leaning more towards Boston.

Overall, many of the large cities in the US would have seamless urbanity on the scale of the busiest areas of Chicago, DC, SF, Philadelphia, Boston, Toronto and Montreal. Still nothing resembling New York.