well, with about 100 shorthauls a year for several years in a row I believe I have some practical experienxe which says that these are biased. I.e. some leisure-Y heavy routes are very tough to get upgrades (BCN for example) and some J-heavy too; interestingly LHR too although A350 but seems that pax have learned the game here as the J cabin is always 100% packed, while some other routes seem to have good availability just because I see loadfactor being low onboard