Where does Li-ion plateau on that chart? The two preceding technologies (NiMH, NiCd) also "doubled every ten years" until they didn't.
The plateau seems to happen once a technology is no longer the winner, and research moves on to the next.
That's because product designers used the improved performance to reduce package volume and weight.
You clearly never used a NiCd camcorder.
I'm guessing Li-ion batteries were always the battery tech used in smartphones, so it might have been harder to notice. I know once I put some Li-ion AA batteries in an old camera I realized I may never need to replace those batteries again.