This would have nothing to do with the effect shown. Paid clicks (i.e. the number of actual clicks on ads) have been growing rapidly over the same time period.

This would have nothing to do with the effect shown. Paid clicks (i.e. the number of actual clicks on ads) have been growing rapidly over the same time period. Dropping CPCs are pretty obviously a result of the mix shifting to mobile (particularly since both mobile and desktop CPCs are rising).