Not to contradict your overall impression on Baltia (that is reasonable, I think), but 'some-other-airline-would-take-this' argument is not true. US carriers cannot do that as Baltia is currently holding the route rights. And Russian airlines are very 'Moscow-centric', sometimes even offering to fly from one Siberia city to another with connection in Moscow. Sad, but true...
St.Petersburg airport has its own statistics on passengers boarding for NYC, and the pax number is fairly big - as one Pulkovo official told me over a year ago, 'enough to fill three weekly flights' (and since then, the airport enjoyed 20% passengers flow growth both in 2011 and 2012). Yep, the traffic seems to be highly seasonable with good load in the summer and mediocre (at best) in February & March, but it does exist.