If there was any danger of mobile phones interfering with aircraft systems then they wouldn’t rely on the general public following instructions to make it safe.
It's annoying that they wont tell us the context of why they prefer it, I'm assuming it's due to the interference I've seen in my own life.
However it could just as easily be spectrum oversaturation, so if 10% of passengers disobey there's no issue, but if 100% of passengers disobey then they have to start restricting phones on planes.
Air travel feels draconian enough, I'm willing to just comply, since this compliance benefits my battery-life anyway I'm not going to split hairs. I would like to bring water with me though.