Good one. But as a non pilot old time flyer, just how high can today's jets sustain cruise in commercial conditions. I remember the early Comet IVs out of Melbourne seemed to fly right on 40,000 feet most of the time perhaps because there was nothing else up there, and it did see 43,000 feet showing on the Qantas 747SPs on the Pacific flights, and more recently I saw 40,000 feet on an Emirates A345.
In fact I've seen 40,000 feet come up on most long range types at least once, but there seems to be some sort of a design or is it commercial barrier to going higher.
I never got to 60,000 feet on Concorde, but I did see it touch 59,000 feet on one flight and 58,000 on another but stay around 55,000 feet much of the time.