I guess the isolation effect is always there, and can be quite strong with even simple in-ear headphones that act a bit like earplugs. Actual earplugs work better though, perhaps it's even possible to use earplugs combined with regular headphones cranked up really loud (though this would be annoying to anyone around you).
What's considered a long flight also depends quite a bit i suppose. To me 4 hours isn't really a long flight, about right to get in the air, watch a movie, have a meal, maybe listen to some music and be landing already. Unless at night i don't really need any noise cancelling gear for that.
It's different with longer (6 hours plus) flights especially at night when you want to sleep and the noise just keeps you up. For some odd reason i sleep a lot better on (even very loud) busses and trains than i do on airplanes - no idea why exactly, perhaps it's the pitch of the sound, the air or something like that.
Last time I was there, met surprisingly few americans actually. The long flight definitely puts people off from visiting south asia. Tons and tons of people from china, australia, and then europe.
I must say that most americans i meet in SE asia are either living there as expats or on a long trip, not people that fly over for a two week or shorter holiday. Perhaps price factors more into that than distance: Flying to say singapore or jakarta is not much further from the US west coast that it is from western europe. The difference is that there is land in between most of the way from europe, and middle eastern carriers like emirates, qatar or even egyptair are fierce competition to direct flights.
In the high season (northern summer) tickets are still expensive, but in the off season you can get realistic flights from western europe to bangkok under $600, and all the way to singapore for $700 or so. The cheaper ones always have a stop somewhere (dubai, qatar, doha, sometimes even in china), but compared to $1000-ish direct flights are often a good option.
The middle eastern airlines are more often getting wifi on board as well (500 mb for $1 on emirates!) so you can enjoy streaming services in flight, which would probably make having some good headphones worth more.