Idk if I totally agree. Yes the cheapest seats start to evaporate a month or so out, definitely agree there.

But looking at this Saturday, there are some 1-way NER tickets from DC Union to NY Penn for $82 still available. Super quick google flights search shows the only airlines that can even come close to matching that involve weird connects/layovers. Plus Amtrak is more comfortable and you can avoid the TSA anal probe.

For last second impromptu trips, I think the bus becomes the real competitor, not flights.