While I understand your general sentiment, no one, not even tesla, has said that the CURRENT batteries are "million mile batteries". The warranty is also different than on the model S. On the model S, it was 8 years, unlimited miles, for FAILURE (not degradation)
On a model 3, its 8 years, 100 or 120k miles (depending on model) with at least 70% capacity left. So tesla is guaranteeing. 70% capacity at 120k miles, not just that the "battery turns the car on".
Depending on what someones actual commute / usage is, if its 70% at 120k miles and even loses the exact same percentage over the next 120k miles, thats 40% at 240k miles. Given that most peoples average commute is somewhere under 50 miles a day, and 40% of 300 miles would be 120 miles, which would still be viable for many people.
I do get your general sentiment though, and happen to agree with it for the most part. I think the battery and motors will outlast much of the rest of the car, actually.