I agree, however Panoramas are more expensive, cost more to run and may be more limited in where they can operate, the staircase on the Elite isn't much of an issue either, it's only the Interdecks people (at least up here) complain about, part of the reason being because sections of the route operate hail and ride, so passengers have to use the stairs while the vehicle is in motion to tell the driver where they want off.
You also have to consider what the vehicle will do in later life, generally Stagecoach cascade their coaches north, Panoramas would be unsuitable in much of the Highlands due to high winds and their size. They've already had one topple over on the A90 just a few months back. Additionally Panthers/Elites will be far easier to sell second hand than Panoramas too. It'll actually be interesting to see what Stagecoach end up doing with all the Interdecks, there's nowhere for Stagecoach to really cascade them to and while there may be a second hand market for them it's fairly niche.