After almost a complete decade I'm down to 78-84% of the original battery capacity. Before buying a tesla, i investigated the cost of replacing the battery out of warranty though Nissan. The cost estimate i got on the phone was over $14k.
That's for a battery about 1/3 the capacity of a tesla battery.
Tesla has a reputation for their batteries catching fire less than their competitors... not letting people hack up their batteries has a large part of that. Allowing 0.5% or however many people ruin their battery risk their own safety for cash would impact the perception of tesla battery safety far more than 0.5% so it is obviously a smart move to preserve their reputation, in a manner consistent with mclaren insisting on applying their own special glue in Woking to preserve the reputation of mclaren roofs not coming off.
Ferrari and McLaren cars are a very exclusive club. Ferrari, to my understanding, explicitly limits production capacity to maintain that exclusivity. The practices of these two companies are in no way comparable to the practices of a large-volume automaker.