Are you within 3-5 m of the aircraft cabin wall, or floor? Probably.

It's not just the gizmos up in the cockpit that may be subject to interference. There's cabling everywhere (overhead, below the floor, in the walls), as well as remote-mounted electronics (not in the walls, but sometimes below the floor). There are also critical components - things like the control surfaces - outside, potentially within a few meters of some seats.

While I suspect the chances of a bluetooth device interfering with some of these seems pretty remote, when you're sitting on a modern jetliner, you are pretty much surrounded by wires and devices that are carrying signals. Some may be a few meters away, others only centimeters. There's at least some potential for interference with any of them. How much potential? I dunno, I suspect it's extremely low - despite admonitions to always switch to airplane mode, you can be sure that there are tens of thousands of mobile devices that are not switched to airplane mode during critical flight operations every day, and planes don't seem to be crashing from that. But there's a lot of uncertainty. Seems to me that complying with the rules is not too much of a burden.