I am sure there must be derailleurs out there which can handle the 42 cog---1x11 bikes must use Something. And I would bet, in time, someone will come out with an Xtra-Long Cage rear derailleur; supply comes from demand, and really all it would be would be to add a fraction of an inch to two derailleur cage halves.

Cost, on the other hand ....

Seems to me it would be a lot cheaper and simpler to go to a triple chainring.