You’d probably need to build a lot more nuclear than we have…but notice how quickly political resistance to nuclear energy fades in the face of actual energy scarcity? Sure, fusion would be nice - as would a step-change improvement in energy storage, which could do the same thing for dispatchable power. But we don’t need any improvement in energy technology from what we have now to keep modern society humming through a transition away from fossil fuels over the very long amount of time that we have available to us to do so (if we want). Long before we start running out of oil and gas, we can have the nuclear infrastructure (plus renewables) to keep us going swimmingly. We’re not limited by natural resource scarcity - only our choices.
To that list I’d add that conventional nuclear has popular support in general and robust support in Congress. If we really needed to or wanted to build out more nuclear there are no real barriers in that regard. SMRs are probably 20 years out, but have some promise as well. Similarly, we could built renewables faster than we are currently doing if we really needed to.
Technology to find and extract gas and oil is constantly improving. And of course, efficiency is improving, and there is a lot of low hanging fruit in that area as well. I just don’t see long term constraints on energy.