I suppose it depends how far in advance you buy.
I recently booked a J trip on Delta to Italy and Holland over Easter for $1260 all in.
I agree entirely. Advanced J fares can be pretty cheap. Not as cheap as JFK but still cheap. I flew to India with BA for less than $3k and just yesterday was shopping around for a trip to SIN next Summer and found an AS/SQ fare in J for only $2800. I know these are not TATL itineraries but I've seen TP fares in the low $2000's for TATL from BOS.
I don't buy this theory that B6 fares will be cheap. I don't see B6 going into this expensive TATL venture and then leave money on the table. they will charge what the market bares which is essentially what everyone else is doing. To extrapolate what happened domestically and expect the same in the TATL market doesn't make sense when the TATL has so much more competition already. We have 10(?) airlines currently serving the TATL market from BOS, not including TK. That is twice as many airlines than the number serving the TCON market from BOS, and TCON is a much larger market.
Using google flights I found BOS-YUL-PVG-TPE-YVR-SFO-BOS for $2500 in January and UA gives you 250 credit if you get the credit card during booking. Tempting to use TPE as a jump off to get to MNL/CEB/DGT for a scuba trip.
Airlines flown: A3, AA, AC, AF, AM, BA, B6, CA, CO, CX, DL, EA, EL, IB, LH, MI, MQ, NH, NW, NZ, OU, PE, QF, S4, SQ, TP, UA, US, VS, WE, WN
2019: CX BOS-HKG, WE HKG-HKT, CA HKT-PEK-EWR, B6 EWR-BOS