The reasoning "batteries are expensive" is circular.
As Musk said, technology doesn't advance as a function of time but as a function of R&D investment. See atomic bomb or space race for the most dramatic example of how throwing money at the problem compressed the timeline.
Car batteries did not get cheaper because 2 years passed but because Tesla and Panasonic invested 5 billion dollars in a giant batter factory. And by creating a demand for lots of batteries, there's incentive for many others to invest in researching better batteries. And before that, cellphones and laptops were driving investment in battery research.
I see no reason why all this couldn't happen 10 years ago if car makers invested a sliver of their profits into electric car R&D instead of, you know, putting it all in Diesel and then writing software to cheat emission numbers.
And as a corollary, if Tesla didn't push the whole field forward with giant investments, BMW would be "objectively" saying in 2030 that batteries are too expensive to make electric cars viable.