It's neither realistic nor unrealistic, we don't really know because it hasn't been done. What we do know is that it won't be cheap. France for instance plans to invest 20-25 billion per year to keep its fleet running. But they are all in on nuclear and basically have no way out. Decommissioning of their 56 reactors would bankrupt their electricity sector so they're kicking the can down the road. The rest of the world will have cheaper options.
This sounds more like anti-nuclear propaganda. Decommissioning a functioning nuclear reactor doesn't need to be more expensive than running it. You just unplug it, let it naturally cool down, and later you can figure out how to recycle the unspent fuel (or bury it deep enough, if people really demand it) at your leisure.
And if renewables really live up to the hype (I kinda hope it does, because otherwise it will get rather awkward for the climate), France can decide to install those fancy always-available renewables at that time, without going thru the pesky "We're being praised for our renewables while burning so much coal and gas!" stage Germany is famous for.
