Your stance assumes that no other parts of the MCU fail that can't be repaired. This isn't the case and, like all electronics, eventually it costs more to keep old tech going than it's worth at which point newer tech replaces said older tech. Eventually, over time, this is what we will see happening as we can probably assume Tesla isn't had hasn't made any new MCU1's in a number of years since shifting all new production to a single unit saves cost.
As I stated previously, I don't see that we're at this point yet but we will eventually be at the point where MCU1 replacements are upgraded to MCU2 (refurbs where possible) units out of necessity. The start of this will be random MCU1 failures being replaced with MCU2 units at Tesla's discretion which we've already seen. The frequency of this will ramp up as fewer and fewer MCU1 refurbs are available until finally they're wiped out and every replacement is done with MCU2 units.