I think Tesla's battery improvements are going to be primarily focussed on reducing cost rather than improving customer's features, and there's also room to continue improving the chemistry in the current cells alongside Tesla's next generation product.
I think the 1 million mile battery is primarily useful for Robotaxis, Semi and grid storage.
Is it reasonable to think that the limited number of Semis reported to be slated for delivery in late Q2 (as linked in this thread earlier this week) could be incorporating Tesla's new cells and/or production methods? The timeline lines up reasonably well with the expectation that Battery Day is being held at such a time that any production would be taking place fairly soon thereafter.