If you buy a battery chainsaw and only use the batteries in that one tool, yeah the batteries degrade. In my case I have a ton of M18 tools which I will always use, M18 shop vac, M18 mower, M18 trimmer, Drill, Driver, SawZall, SDS, Flashlights, Compressor, Circular Saw, etc etc
So realistically, the batteries need to be swapped eventually regardless, which makes the battery degradation for just the saw, "free" so to speak
If you use the saw a lot, I don't see how it just keeps going further into the battery saw's advantage, thats assuming you have a lot of batteries, good chargers etc, of course
I could run the saw near endlessly