First it should be said that this thread is primarily about decomissioning existing nuclear power plants. It makes enormous sense to keep operating those plants until we have a world like the one you describe, regardless of how much newer plants would cost.

But more importantly, your assumptions about the future are very optimistic. I'm sure the Germans also thought they were being very smart when they decided that nuke capex was not worth it because gas was so cheap and easily available, and then now we are finding out that this decision crippled their economy because it caused a dependency. In my opinion throwing all your chips into a technology that requires materials and production capacity you don't have, and in some cases doesn't even exist yet, is a real sucker's bet. All your rosy scenarios would fall apart in one second if China decides to stop selling batteries to you.