The thing is, DC didn't end up being that big a gateway for the nation's future expansions to the west, now did it?
I'm not entirely sure, but it would certainly appear that an 'up the Susquehanna, overland, then down the Ohio' route made a more sense when it comes to trekking west than the 'up the Potomac, overland, then down the Ohio' route. It appears that it was easier to cross the mountains along the former route, and for this reason, it appears to have been more commonly used by people heading west. (On the other hand, on a map at least, the Potomac-Youghiogheny-Ohio route appears very sensible, because the distance between the source of the Potomac and the source of the Youghiogheny is so negligable. I'm not sure how sensible it was in practice, though. Maybe those rivers aren't actually useful in any way, that close to their sources?)
Assuming that the 'up the Susquehanna, overland, then down the Ohio' route is indeed the best option, then a capitol on the Susquehanna actually has a far better shot at becoming a gateway for the nation's future expansions to the west. It would objectively be the better location.