Because solar panels degrade (down to 80% aber ~25 years) and wind turbines are also only engineered for ~30 years.

You could use them for longer, but it usually makes more economic sense to dump/recycle them and put up new ones, which usually produce quite a bit more power.

The nuclear buildout already really slowed down before for economic reasons. Three Mile Island certainly had an impact, but it was only a part of the reason why the US stopped.

Why do you think Lithium is running out? There is enough Lithium that could be mined if there was enough political will (and sometimes price signals).

Besides, we might also use other battery types, such as Redox-Flow for long term storage.

The current “backup” plan is to produce Hydrogen or P2G continuously, store it, and use it in cheap, inefficient gas turbines for the week every few years (without large grid interconnectivity) where there is a prolonged Dunkelflaute.

We did build a lot of nuclear in the 60s and 70s, but prices didn’t really drop and began to increase (higher safety standards, more oversight, general cost disease as is usual for large civil engineering projects), so we stopped building nuclear. It also wasn’t sustainable to build nulear at that rate for some countries, as, well, after you built enough you don’t need more (france) for a while.