Don't assume your case for everyone... Some batteries can be defective...
I bought a Nvidia Shield Tablet 4 example and got a 2nd one for free from Nvidia because the first one had a defective battery (which was actually a general problem on tablets using that exact battery), same goes into some mobile phones that are still having a series of defective batteries...
Anything can happen.
Also heat reduces battery life a lot, so if the battery is getting a lot of heat from the Switch cooler while docked, it will surely have its life also reduced by getting heat continually, same when using as portable and with too much power being draw from battery which heats up battery.
ps: Anyway on the defective Nvidia Shield tablet, I have seen people that simply changed the battery and got 2 fully working Nvidia shield tablets (I still have my old one working and like new), AND I ALSO SAW SOME PEOPLE REMOVING THE BATTERY SOLDERING DIRECT POWER AND ATTACHED IT TO THE BACK OF THE TV AND USE IT AS A MEDIA CENTER like the Nvidia Shield TV.