I've spent a long time in that community, and a golden rule is to never mix cells - even cells of the exact same type because they're often binned to be matched to be even closer in operating capacity. Also, and more relevant here, mixing old and new cells is also a big no no due to changing characteristics during aging and use.
You can't just pop open a battery pack and replace a bad cell. I mean, you can, but if you cause a fire that kills someone, you should go to prison for negligence.
Many manufacturers make this difficult or impossible on purpose, and apparently they also have lobbied to change legislation to continue being able to do this, using battery fires as an excuse to cement their parts and repairs monopoly. It's that simple.
You then want to extend that to the other main parts like the motor or the controllers, so you're not left with a piece of junk when one component fails. Just like for normal bikes.