That sounds like a ridiculous assumption. Airplanes are basically guaranteed to not discharge the batteries lower than 20% for safety reasons. I’m guessing they’ll also not charge to fully 100%, like some BEVs. On fixed routes they could charge only what they need to get to where they’re supposed to go plus the required buffer for emergencies. The batteries will probably also be cooled. So it could be an ideal scenario for battery degradation. There are 10yo Nissan Leafs on the road right now that was pretty much the worst case for battery degradation.

You can also imagine that planes will start operating longer routes and then move to shorter routes as the battery degrades. Since the batteries are large they could get a decent amount of money for them when they’re too degraded for airplanes. They should still be useful for energy storage.

I also think it’s likely that when airplanes go mainstream, they’ll use a different chemistry than the standard Li-ion chemistries we have today. Maybe solid state lithium (Quantumscape?) or sodium ion. So it’s very hard to say how big the degradation problem will actually be.