It comes down to energy. Fusion would dramatically decrease energy costs, which would in turn drive costs of almost everything else down and allow us to do things that had been only a dream before such as fusion propulsion. So run your cost/benefit analysis again with that in mind.

This is even before we get into ecology - most modern energy sources are highly pollutant (oil/coal). How much better off will planetary ecology will be if fusion comes online 10 years earlier?