The trick is the battery: When a cell blows in a string that string is dead. Lose enough strings and range suffers. I don't know how long tesla packs last, and I think they have spare strings in there which are switched in as older ones blow to maintain range. Which is why they can turn on "escape range" in disaster areas. However the design is closed, so finding out what's going on in the pack and in the electronics requires a lot of reverse engineering (which is way difficult if they encrypt the code on the chips, which they probably do).
So... hm?
There are no spare strings, only some models have a software-caped battery capacity that is temporary lifted in disasters. The battery management is closed but it's not too difficult to find a bad cell. The problem is that all the cells are glued to the cooling system and replacing a cell is next to impossible. They do put many cells in
series paralel, with a fuse wire on each cell, so a misbehaving cell does not kill the battery, only reduces the capacity. (basically, if it shorts itself, the fuse of that cell is blown by the charger or all the other cells dumping there load into that cell.)
Tesla 3 has just 4 battery modules that could possibly be replaced, 2 long modules in the middle, and 2 shorter modules on the sides to give the front wheels some space. The trick to long lasting li-ion is keeping them balanced, both by tight manufacturing tolerances and good battery management electronics, and careful regulation of battery currents, voltages and temperature while charging and discharging.
Current Tesla batteries won't last 1M miles but that is what they are aiming for, including the batteries. How many car manufacturers are aiming for that kind of lifetime without major maintenance?
Good luck with your local repair guy fixing that thing. 
