We should be careful with apply Moore's Law style of thinking when it comes to battery innovation. These things can and will hit hard limits at some point. Real progress can be much slower and much less predictable than in the semiconductor industry.
The funny thing is that it really hasn't been that much less predictable. Progress has been slow but steady and Tesla's business model has been built around largely predictable technological progress.
7.5%/year is probably an overestimate, though, and you are right that at some point it will hit diminishing returns. Moore's law has too, though, eh?
“Every year the energy density of batteries increases by about 7.5%”
So, in 10 years electric cars will double their range, which should be more than enough for most people.