You are half right and half
You are half right and half wrong. Yes breakthroughs don't happen often, even when they do, by the time they are commercially viable they end up seeming much less of a break through as the older tech closed the gap. But here is a few things to note that people seem to miss. First of all, batteries are not improving at a slow pace, the issue comes up is people put batteries side by side with computers and other such tech. Compared to CPU processors, yes batteries are improving at a slow pace. But if you put it in context of everything else, batteries are improving at a fairly fast pace. A lot of people are too obsessed with a breakthrough, but the reality is a breakthrough is not needed. With incremental improvements we can have cars on batteries going 500 miles on 1 charge that can be recharged in 10 minutes. Without any need for a breakthrough. The biggest thing holding back EVs is not that though, it is the cost. And this is what Musk is tackling, to bring down the cost so that eventually that 500 miles EV will be affordable. For now though he is starting with affordable 200-300 mile EV (Tesla said there will be options for larger batteries from 200 miles on the Gen 3)