1. If low online CPMs were a result of oversupply, why haven't CPMs in other media dropped by the same amounts? If oversupply were why prices are so low, not efficacy, then online media would be a substitute for other media and ad spend would shift to the low-cost medium, equalizing prices. This is not what seems to be happening.
But if online ad spend has not yet caught up with online attention, that supply is temporarily exceeding demand. One way to test this would be to see if offline CPMs are increasing (because demand there would exceed supply, driving up prices.) If it were true, it means that online CPMs should increase over the next few years back to "normal" levels.
2. Since media sells attention but is paid by the impression, then the fragmentation of time spent on media might lower CPMs as the "bad money" of low-attention impressions drives out the "good money" of high-attention impressions.