My hope is that AGI effectively kills scarcity and therefore capitalism, and without capitalist pressures, I think a global transition away from fossil fuels entirely will be pretty easy

In some parts of the world, we have already reached that stage. For example in the US, everyone has enough food, energy and entertainment. But guess what is one of the biggest source of discontent? Housing, which is indirectly tied to land via zoning laws. Capitalism or AGI cannot really solve the problem of limited desirable land and politics, unfortunately.