This is an historical artifact. When that generating capacity was built, nuclear was the alternative to fossil fuels. It isn't today; renewables are now cheaper and faster to install.
Your argument can be seen as a way to ignore the cost decline of renewables without at first glance doing so.
(As you say, there are hydro-dominated grids with no nuclear, for example Costa Rica, which gets 98% of its electrical energy from renewables.)
Comparing to renewables is a different story - it can be done but it's much more complex (not dispatchable, seasonally variable) but certainly renewables makes sense in some or many places, (and in all places for a portion of the supply). I'm all in favour as long as it is actually done with the intent of getting to zero CO2, and doesn't just stall when it gets to the more difficult part of the transition. So many places get to 50% annual generation as renewable and throw up their hands about the remainder and leave it on coal/oil/natural gas
It's comparing nuclear to alternatives, one of which is natural gas. Others are renewables. One cannot make or justify a decision on nuclear without considering all the alternatives.
From point of view of an electrical grid operator only wind/solar+fossil are alternative to nuclear, if you want to maintain stable electric grid without blackout or rolling blackouts.
This is a common misconception. The reality is that coupled with storage in a cost-optimized way renewables are no more expensive than nuclear for providing 365/24/7 power and likely considerably cheaper.
I should have clarified it more precisely:
Weather dependent renewables (wind, solar) without storage are not an alternative to nuclear.
Now we can discuss more precisely the kinds of storage: pumped hydro, battery storage, H2 storage, thermal energy storage, the costs and space and material needs of each kind of storage.
This energy model is not 100% renewables, Combined cycle gas turbine with CO2 capture is also included in the mix.
This optimizes cost because different properties of the storage drive cost in the two cases. For the first, there are thousands of charge-discharge cycles, so high round trip efficiency pays off. For the second, there are many fewer such cycles, so efficiency has a much lower payoff; instead the name of the game is minimizing capex.
In practice the second will not be installed until later in decarbonization, especially if the storage is being used to counter rare dark-calm periods when neither wind nor solar are available for a prolonged period. But it can be done, and so little of the total energy flow would go through in that case that high per-kWh costs for that small fraction can be tolerated.
Anyway, your clarification destroys the argument that was being made that nuclear is needed.