There's going to be a point where every tesla has a shot battery just like how that happens for literally all our technology with a battery in it. EV isn't any less prone to degradation than any other lithium battery, same old tech as your phone battery or power drill. At that point either you throw 10k into the car or its a 4000lb paperweight. Basically, these cars are going to be totaled and scrapped a lot sooner than traditional cars.

I certainly would think twice about an old car with over 100k on the clock for $20k. There are plenty of other components besides the drivetrain that can go south on you and are expensive to fix relative to the value of the car. Some ICE cars are notorious for the drivetrain going 350k+ miles while the rest of it might rot apart. Honestly modern cars are terrible to drive when they are old. My old 20 year old honda had a leaky sunroof, leaky tail light seals, slow window motors that cut out in the cold winter, every hydraulic line corroded and leaked at one point or another sometimes multiple times, frame rust, shot rubber in the suspension components, shot brake system, rubber and plastic body trim that was just cracking and sloughing off from 20 years of UV bombardment, blown speakers, and a motorized radio antenna that never once worked. The drivetrain on the other hand was flawless being a honda, just needed an oil change with synthetic once a year ($35 or so and 15 minutes of my time mostly passive waiting for it to drain) and it was still on the original clutch.

With a tesla you still have suspension components with rubber. You still have rubber gaskets and seals around windows and doors. You still have hydraulic brake lines. You still have steel components that will rust. You still have electric motors controlling things like windows (and presumably the sexy door handle release) that probably have plastic gearboxes. You aren't out of the woods, far from it.

Theres a lot more than the drivetrain that goes bad on an old car. In fact depending on the sort of car you have the drivetrain is the absolute least of your problems and might be able to go around the world 10 times.