We'd obviously be in a far better position climate wise if a lot of electricity generation in the 80s forward had been from nuclear rather than fossil fuels, and if we do not prematurely shut down nuclear reactors. (Though I think this mostly happening in Germany and Japan only? In the US our nuclear fleet is aged, and there are few life-extensions that make economic sense when compared with the current cost of renewables firmed with storage.)
However, I think the picture of how rosy a nuclear future could be was oversold, even in the last century. Construction performance in the 1970s was abysmal. Even the most successful deployment, in France, stopped short of its initial plans, and did not fully decarbonize the grid.
Starting new deployments of modular reactors means talking about new construction modes, modes that we have no idea at all if they will be effective or cost efficient. It's a huge gamble, that even if it pays off, won't start paying off until 10-30 years from now. Nuclear is not a technology that we can depend upon to save us now. Which is not to say that we shouldn't try, it's just that we need to have backup plans in case Nuclear can't be built.