Both have to be plugged in. And there is no benchmark about the aging of the battery. When the macbook pro battery got older it will be much earlier empty. The swapping of the battery i a pain on apple laptops. For most otherlaptops it's pretty easy. Only a power cable detaching and removing a strip or screw mounting than its out. For mbp you have to remove the trackpad also.
Not a pain, I swapped a battery in an Apple authorized repair, they ordered it from Apple and when it arrived, they swapped it in a few hours. No pain at all. I am not going to remove strips or detach cables, I’m not a student any more, I do professional work on my Mac. If something is wrong with it, I’ll bring it in for repair and let people who are experts take a look - and I can even get a replacement Mac if the repair is longer than a day or two.
And, btw, the laptop I had the battery replaced was my wife’s Mac after 6 years of solid battery life.
So. How long are we going to be here and make stuff up? If you prefer Windows laptops, go and get one, by all means. But this attempt to present these computers as overpriced, underpowered or whatever - that’s just silly. These are, currently, some of the best laptops on the market. If not
the best.