Degradation always occurs. "Over" charging and discharging can accelerate it, but it's always going to be the "over" due to the pressure of packing it full of more energy.

Same as any battery, and also why smartphones with fast charging only do so up to a point. It gets hot, fast because it requires more to pack it in the last ~20%.

Heat is the biggest factor, and that is more common on the high end.

Unfortunately this is one reason of many why our current battery technology is not so sustainable; they just don't last.

Within the first couple years of owning a phone for example you will have lost a lot of capacity for your battery. This suits businesses because then you'll trade it back in for a discount on a new phone you probably wouldn't have bought for another year or two. But it's bad for the environment and bad for the consumer.

TL;DR: We needed to improve energy efficiency and technologies elsewhere long before we tried to push the adoption of electrical cars, instead of throwing darts at a board with various ways of fucking everyone over.