In net terms, you're probably right. But at least the energy used for cryptocurrency is being used toward something that might benefit many (commerce, hoarding, what-have-you), vs against something that might result in the arrest of few.

The economics I'm thinking of are along the lines of cryptocurrency energy usage per participant, vs image scanning energy per caught perpetrator. The number of caught perpetrators via this method over time will approach zero, but we'll keep using energy to enforce it forever.

All this does is remove technology from the problem of child abuse, it doesn't stop child abuse.