Tesla already has the million mile batteries shipping in cars right now. And that's with mild degradation. It's many millions if you allow 20% degradation.
They don't warranty or advertise it because it would dry up demand for those without the new batteries.
en.m.wikipedia.org
That company went out of business because it upgraded the product with too many improvements. Nobody bought the first, waiting for the second. Cash flow dried up.
Tesla didn't fall prey to that mistake. They kept it on the down low.