Even if we did develop fusion, how long would it take to switch to it and would it even be economical? A ton of places still use coal and other fossil fuels simply because they're cheap or were already built. It's hard to make assumptions about how much fusion would cost when we still don't have it. Nuclear fission's fuel is cheap (cause so little is needed), yet nuclear is still insanely expensive because building the plants is a difficult task mired in red tape and the general public is afraid of it (on that note, I'm not sure how many people even have a concept of fusion beyond perhaps what Spiderman 2 made them believe).

There's also the divide in the world. Even if the rich western countries got fusion, would everyone? The west also got covid vaccines early while much of the rest of the world had to wait. And further drawing on the covid analogy, some countries pretended there wasn't an issue, which already is going on with climate change.