Well you are just proposing to use molten salt as energy storage, but if we have efficient storage we don't need nuclear because we can charge the storage much cheaper and environmentally friendlier using renewables.

As a side note, every time someone points out the shortcomings of nuclear power plants (not easily adjustable, huge investment costs/expensive, storage ...) there's the replies about, "but those new future plants that we are researching will solve all this". We don't know if reality will actually deliver on those promises (nuclear has quite a history of broken promises), especially if we are talking about acting fast.

Yes, this is energy storage, but a big tank to put salt in has the potential to be considerably cheaper than lithium ion cells.

To your second point: all technologies have the property that we can't know for sure if they'll deliver on their promises without trying them, and the very reason we don't know in the case of nuclear is because opponents of nuclear power undermine attempts to try out novel architectures, pointing to limitations in current technologies as justification.

We could just store the surplus renewables in molten salt and don‘t need nuclear for anything.

If what we have is electricity, turning it into heat to turn it back into electricity again is very inefficient. If what we have is heat and we're going to turn it into electricity anyway, storing it as heat for now and turning it into electricity later is much better, and comparatively trivial to do. That somewhat limits the applications, though, with concentrated solar being the only obvious renewable one. Solar PV and wind don't generate electricity from heat.