We could have switched most of the planet to pure nuclear energy decades ago. If you believe climate change is an existential threat, how can you be against nuclear energy?
We are at a point where huge investments in renewables are becoming a reality, but the damage is already done.
Coal and natural gas are incredibly effective in these respects. The plants can be built near the places the power is needed and / or in places where the required grid already exists, and they inherently produce steady power on demand. Nuclear power is a drop in replacement for these attributes that can supplement renewables in a low emission way today.
Without it, or a series of technological breakthroughs in renewables, we'll just end up with an overall mix of renewables supplemented by coal and natural gas -- which isn't going to be enough. And it's not good to bet the world on the hope that we have some technological breakthrough.
Given enough time, we might instead focus purely on solar and wind, but we don't have enough time. We need to get to zero net emissions as soon as possible, and we need to use all options available to us if we want to take climate change seriously.