An analogous situation is electric cars. Electric cars, eventually over the lifetime of use (not immediately), save emissions over combustion engine cars. However, building a single mile of road is something like 10x or 100x (I forget the exact multiplier, but it's at least an order of magnitude and maybe greater) the emissions of a car (electric or combustion). So given the idea that electric cars actually bolster if not increase the love and use of cars, then that has a downstream effect of more roads being built and maintained, which is far more polluting than the cars themselves.
So you need to look at things as a system. I'm not arguing for anything but looking to understand things. I think fusion is obviously something we should do. What I'm wondering is how do we get people to realize it isn't a one-stop shop for staving off even more environmental change.