The cell phone rule has always been an FCC rule, not an FAA rule because the concerns, as stated above, that it would tie up signals from too many cell towers simultaneously.
That being said, occasionally I have an FA push the issue and I comply. Not worth the problem.
And ... it if was an avionics problem, I figure that over the past 10-20 years, the number of individuals flying on planes with their cell phones not in airplane mode has multiplied by hundreds ... yet planes don't seem to be falling out of the air even with all these wifi equipped flights.