Trains are great in general. They also tend to be a poor fit for anything much more than carry-on. I've done it and managed but it's better not to if you can.
I generally agree with you - I spent years flying across the world with a carry-on only and I still miss that lifestyle.
But Japan is the kind of place that people want to bring a whole lot of stuff back - I know a lot of people who basically fly out with empty suitcases and just fill them to the brim with random tchotchkes over here — and hey, whatever makes them happy.
Having traveled to Japan quite a bit, I can definitely see that. Though I'm also at the stage of my life where I do not want anything else to enter my house. :-) (And I have quite a bit of stuff from Japan my dad brought back from when he was traveling there a lot.)
But Japan is the kind of place that people want to bring a whole lot of stuff back - I know a lot of people who basically fly out with empty suitcases and just fill them to the brim with random tchotchkes over here — and hey, whatever makes them happy.