here's a thought based on observations in restaurants over the past month (really heavy duty going out every night) - i don't know that most people go somewhere after dinner (or before). i think that people tend to linger more these days - dinner IS the main event.
I'm a bit puzzled by the after-dinner digression.
if the decor is not fab... and the service... it's hard to be "the event" for any evening
here's the thing: prime revenue nights for NY restaurants are Thursday, Friday, Saturday. in fact, most of them aim to make 50-70% of their revenues on just Friday and Saturday. as Stephen Starr said, "you can't make it without the B&T"...
weekend diners (Thursday is part of the weekend while Sunday is not...for restaurant purposes) are going out after dinner. they've already parked their car or taken a cab to the restaurant...fighting for another cab at 10:30-11 to a nightlife destination is not ideal.
in other words, I think you're confusing the days that serious diners eat with the days that restaurants need to stay afloat financially. proximity to nightlife options matters for destination restaurants (below the four star level).
This is a digression, but it might be interesting to consider the extent to which restaurants have become an adjucnt of nightlife in NYC.
As Wilfrid (I think) has pointed out, to a significant extent, they have. And it's certainly affected the NYC dining scene: you have restaurants that look like clubs, restaurants that base their service models on clubs, etc. But that's mainly true for a younger demographic. I know people my age who eat out plenty, including on the weekends, and almost never go anywhere afterwards. (And the same, I'll bet, is true for their B&T contemporaries who eat out in New York only on weekends [to the extent that I and those of my friends who live in Brooklyn aren't B&T ourselves]). You can't regularly stay out till 5 AM if you have children.
So the question then becomes, are people my age now a negligible portion of the constituency for restaurants in New York? And the answer is . . . it depends on the restaurants. (Surprise!) I think your view of the way things are is skewed by the kind of places you tend to go. You actively favor restaurants that court a younger demographic and are a part of nightlife (as, in certain moods, do I). But if you look at the places my contemporaries tend to go -- places that you'd probably write off as too boring, or not having enough buzz, or having insufficient eye candy -- you get a different view of things.