Right. And it's totally useless (in this context), because even if it did tell you when the battery is likely to fail (which it does not), there is literally nothing you can do about that.
Major fallacy. You can't use the linear phase to project the end-of-life, because at some point it stops being linear.
The important point is that with a battery, there is basically nothing you can do to prevent that. With a gasoline engine, you can, with inexpensive repairs.