By the time countries that could have built nuclear power plants would complete them, they will have collectively burnt enough coal and gas to doom humankind.

So: indeed, the world leaders didn't try seriously.

You could also channel the effort into renewable energy and the the reward would be greater and faster achieved.

Yes, that is exactly the nuclear motto: It's too late to match any climate goal with nuclear power not already starting counstruction many years ago... so let's say "fuck climate goals and stop trying" and start building nuclear anyway, because it's really cool and in 20-30 years it might solve our 10 year problem of remaining co2 budgets.