The underlying mechanic here is that cost projections are being used to sell a technology. As such, there is very strong incentive to underestimate the costs. This applies to FOAK plants and to projected experience rates.
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.