Some a/c do have the range. TG used to fly their A345 JFK-BKK non-stop (17+ hours!). So YYZ-BKK would be possible. Also, Singapore flies an all-C A345 EWR-SIN.

But a problem with the A345 is that it doesn't hold enough pax to make any money on such a long flight. TG's A345 only held 199 in Y (IIRC) and I think it was 28 in Y+ and 60 in C. There was a market for the flight of people who prefer to fly non-stop and I think load factors were pretty good on the JFK-BKK route. But it still couldn't generate enough revenue. The LAX-BKK route is also a long flight that has been served by the A345. There was a plan to change the a/c to an A346 or 777 with a technical stop at least at KIX to refuel. Not sure if that happened yet. They kept on changing the start dates of the plan.