Tesla has vertical integration with their batteries, which is why they can make them cheaply.

> This assumes it is literally impossible for anyone else to reduce their battery costs to a level which makes them competitive with Tesla in an environment when battery prices are falling rapidly and the tech continues to evolve

No - it just means they can't do it as fast as the Chinese. The Chinese have been investing in battery technology for 10-15 years longer than most auto manufacturers. (And their labor is cheaper).