@jws
You need to think beyond the in-flight environment. Bluetooth & WiFi have other uses such as connecting peripherals like speakers, headphones or keyboards. AirPlay & AirDrop also use both at some point but a cellular connection isn’t always needed.
Plus you can speed up battery charging by preventing apps & systems from trying to find signal or background operations such as Mail fetching.
Back in September last year when I was hospitalised with kidney failure, I had no way to charge my iPhone for two days until a friend could bring a charger in while visiting. I used Airplane mode to eke out my remaining battery power by shutting down cellular, Bluetooth & WiFi; only activating cellular to get updates from the outside world or to contact social media.
// @lograh