Every long-haul flight has a moment, somewhere over an ocean, where someone taps their phone twice, frowns, and asks the person next to them whether their eSIM is broken. It isn't. Here are the myths, retired one at a time.
Myth: your eSIM keeps working once you're airborne
It doesn't, and this has nothing to do with eSIM versus physical SIM. Flight mode switches off all cellular radios in your phone, by regulation, the moment you enable it before takeoff. An eSIM profile sitting dormant in flight mode is doing exactly what a physical SIM would do in the same drawer: nothing. The technology isn't the variable here.
Crew ask you to enable flight mode for the same reason on every airline, on every aircraft type, regardless of which network or which country's SIM you're carrying. If your phone genuinely showed a live signal bar at cruising altitude, that would be the story worth writing about - not the reverse.
Myth: onboard wifi runs on your eSIM data
It doesn't touch it. Onboard wifi is a satellite or air-to-ground internet service sold directly by the airline, entirely separate from any mobile network subscription, travel or otherwise. Buying wifi for the flight and buying an eSIM for your destination are two different purchases solving two different problems - one three hours long, one ten days long.