Bigger batteries charge faster than smaller batteries of the same type (structure and chemistry). Tesla batteries seem to be made of multiple strings of batteries wired in parallel. The strings consist of batteries wired in series to produce 400 volts. Each string can absorb a maximum amount of current when new. As the batteries get older and experience more charge/discharge cycles the ability of the string to accept charge current and deliver discharge current declines. Tesla says the batteries will maintain 70% or more capacity after 8 years.
When capacity goes away it reduces the maximum charge and discharge current. Charging current times voltage equals the power that the battery can receive. Reduced Supercharging power levels is a direct consequence of reduced battery capacity. So, if a new battery could accept 120 kW charging then an 8 year old battery should be expected to accept 70% of that rate which is 84 kW. Attempting to charge at higher rates will result in energy being converted to heat rather than being stored in the desired chemical reaction. Placing a limit on charge rates seems to be the right course of action because it protects the battery and reduces the potential for overheating. The battery should last longer if the charge rate matches the capability of the battery.
Electric vehicle manufacturers should be telling their customers that battery capacity and charging rates will drop over the life of the vehicle and providing a yearly minimum capacity warranty. A battery replacement option should be triggered at any point when the capacity and charge rate drop below the level that the manufacturer has guaranteed at the time. So, for example, if a battery drops below 85% after 4 years and the manufacturer has guaranteed 85% or better then the owner should have the option to receive a new battery at a pro-rated price. The number of high current charge/discharge cycles or distance travelled might also be the basis for determining the warranty. People are used to time and distance for automobile warranties so manufacturers might find these metrics to be more familiar and therefore more acceptable to consumers.