That was my original thought as well, but reading the article again, it looks like they are talking about putting 5G "cells" on the actual airplanes. Given how much cheaper satellite broadband access has gotten, it actually makes a lot of sense.
Cell phone providers will negotiate with airlines to get their "micro-cell" installed and connected to the existing internet uplink. Once this is fairly ubiquitous, cell phones will be "whispering" to the in-airplane tower, and hence won't bother the cell towers on the ground. A cell provider's goal is to provide access in as many places as possible with as little work required by you as possible - this is one step better than inflight wifi from that regard.
Lufthansa runs a micro-cell onboard the aircraft. You're connected to a single dedicated cell tower inside the airplane, not to stations on the ground. No handoffs involved.
(The airplane's micro-cell will then have a backhaul over satellite or dedicated air-ground radio, just like in-flight wifi. Yes, there are handoffs there, but it's not via the normal cellular network - which is an important distinction.)
This also means the cellphone's radio is transmitting at minimum power, since it's right next to the antenna for the micro-cell.
Satellite connectivity is no longer expensive, so why would you make it prohibitively expensive when you can also offer a $10 in-flight internet option many passengers will use?
Because you are a telco and exploiting your captive market is your business model. The same reason why an out-of-plan international call can cost dollars per minute unless you happen to have the specific, convoluted combination of plans/add-ons/bolt-ons in which case it magically becomes free.
Who wouldn't want the opportunity to silently rip people off for several bucks because their phone accidentally connected to the in-flight cell and dared to transmit a few kilobytes of data?
Even from your perspective, telcos would have a utilitarian need to price attractively, in order to change both consumer and business expectations to be connected and to persuade them to give up downloading content and work in advance of the flight.
That doesn’t seem to be happening though - there was plenty of opportunity for this in the EU but it took legislation effectively mandating roaming to be free for it to become affordable - until then carriers were happily swindling their customers.
Unless they’re talking about the aircraft itself providing the 5G network and that uses a probably-prohibitively expensive airline data plan.
I doubt Flight Mode would disappear from phones, it’s useful for situations where you want to conserve battery like camping.