As much as I love Philly, I'd have to say it's probably not the most populous city in the UCAS. Seattle is probably #1, but Philly's definitely in the top 5, if not top #2.
Seattle is at, what, 3 million in 2072? That's five times the population of today, but understandable since much of the Northwest flocked there and then it became a boomtown.
The top five of today are, in order, NYC, Los Angeles, Chicago, Houston, and Philly.
New York went through the quake and lost Manhattan. LA was California Free State but now makes up about half of the PPC population. Chicago flatlined. Houston's in the CAS. Philly's the only one that hasn't had a disaster.
(The next five, again in order, are Pheonix Arizona, San Antonio, San Deigo, Dallas, and San Jose. Indianapolis is the third largest USA city still in the UCAS.)
(Oh, Toronto crops up above Indy once you add in Canadian cities.)
I boosted Philly from the current 1.5 million to around 6 million, drawing on refugees from New York and, later, from Boston, but, Rotten Apple didn't exist at the time and that could change things on my end.
So, the top four in 2072, but NOT in order, are New York City (Which, minus Manhattan, you almost expect them to fracture into four normal-sized cities, but that goes against the Metroplex idea), Philly, Seattle, and Toronto. Indy may well be #5.
...
Not that this exactly means anything, but hey. 
Actually, I have to differ with you on city ranking today. You're right if you use the incorporated area. For a number of reasons I think MSA ranking is better. That is, what's generally called the Greater Metropolitan (city of choice).
The top five today in the US are NYC, LA, Chicago, Dallas, Philadelphia. The next five are Houston, DC, Miami, Atlanta, Boston. Seattle is fifteenth on the list. If we add Canada, Toronto would fit between Miami and Atlanta while Montreal is almost exactly the size of Seattle.
Old topic I know, but I'm a stickler for details and a proud Torontonian. The Greater Toronto Area is larger than Miami, only 30000 less than Houston at 6050000 roughly, MSA is an American thing, so naturally we do not use it in Canada, Toronto is really only the city with the one mayor, going by that standard it would be twice the size of Dallas, nearly three times. The GTA includes a few closely tied cities that still have their own Mayors, like York Region and Mississauga. Lots of people in these cities work and visit Toronto regularly, the bus routes overlap and there are ways you can only pay a single bus fare to get across the entire area. York University is actually in Toronto (south of Steeles Avenue, the border) and the subway is getting extended up into York Region, Mississauga also has a University of Toronto Campus as does Scarborough (which is now incorporated into the city proper).
Also, with the NAN popping up I can see Toronto getting a large population boost, les Anglais from Quebec (which has happened before, when the Seperation movement was associated with terrorism and vandalism) and Anglos from Athabaskan, Algonquin and Salish. That puts Toronto as the second largest city in the UCAS probably, figure 10 million easy.
