Going back to the x² reward curve was a mistake

That never happened. What are you talking about?

which the √x reward curve would do

No sublinear curve can possibly work. It would be attacked by milking votes being split up across spam and small rewards. The curve must be at least linear or mildly superlinear.

Linear reward was a step in the right direction

Curiously, the graph you posted showed that the decline was pretty much the same with linear rewards once the crypto boom of late 2017 (which was certainly externally driven) had played out. Likewise earlier in 2017 before the boom with linear rewards (also declining user base). And for that matter we could probably say the same about the n^2 era.

The curve really doesn't appear to be the major factor here