I kind of wonder if swapping out the drivetrain is actually a really common and perhaps early part of Tesla's troubleshooting procedure. Since it's just a motor, batteries, and relatively simple transmission it probably isn't nearly as hard to remove as a combustion engine (especially if they designed it for easy removal). It's probably a lot easier to pull the drivetrain out, swap in a known good one, and have the customer on their way than spend hours and days trying to debug the unit in the car (and potentially having the customer back again if the issue isn't resolved).