Are they not replaceable already? Older Teslas could literally change the battery in a few minutes. Newer ones aren’t quite so easy, but it’s a fairly quick matter of lifting up seat cushions, undoing some bolts and connectors, and connecting the new one. If your battery dies under warranty, they’re replacing the battery, not giving you a whole new car.

The issue isn’t that the batteries can’t be replaced, it’s that a new battery is quite expensive. Substantially more than a new motor for a typical ICE car.