Paying per computational step could mitigate distributed denial of service attacks (DDoS) and abusive spam when launched against a large number of targets at a near zero margin cost. The potential benefits of this pay-per-computation model for the mitigation of network abuse cannot be understated.
What the debate around bitcoin is currently focused on is not what is sustainable or how urgent the need to increase the blocksize is. The debate is around what the precise use case of bitcoin is - an ideological crisis, not an engineering crisis. A vision crisis, not a management crisis.