I would think that in any aircraft with 100+ pax, there will be at least one cell phone left operating for the entire flight. Pax boards goes to window seat pulls down blind (de-rigeur these days) straps in and goes to sleep phone operating. As far as I am aware no modern cell phones have been shown to interfere with aircraft systems: not the cellular signal, the WiFi, nor, the Bluetooth. It is not uncommon for the aircraft WiFi to be on at all stages of the flight (although the onward network connection to Internet may be offline) the aircraft base station is a LOT more powerful than a phone WiFi. I think that this is one of those rules like the ban that used to be applied on cell phone use at gas stations where nobody wants to be the first to say it is not necessary.
At the same time as the flight attendants are attempting to enforce a rule that has no safety impact, they are not asking for window blinds to be opened for taxi/takeoff and landing that does have a safety impact should an incident require a decision on what side NOT to evacuate from for example.