This is a timely post for me! I'm struggling with my beloved Tri-Star-- I'm still packing too much junk in it but can't figure out what to sacrifice. I've never been hassled about it, though. I really, really, really want the smaller Aeronaut that probably won't happen, so I'm trying to decide whether to move up to an Aeronaut. I suspect this new camera insert might propel me to pull the trigger on buying the Aeronaut.
I agree with you about the handiness of fitting the TS under the seat of a 319/320. My second flight on the recent trip to Italy was on a 319 and it was so handy to slide the TS under there and stow the Imago in the overhead bin (I had a window seat in an Air France/ Alitalia "premium" economy section.)
I felt hugely overpacked with my bulging TS, but still felt light compared to other travelers-- and I have to say I felt a bit smug with my handy backpack in Venice and Florence, with all the other travelers pulling giant wheeled monstrosities. I think if I ever am not able to carry a backpack, I'm going to switch to carrying one extra dress, a comb, and a toothbrush in my purse. I never want to have to pull wheeled luggage around Italy!