MSRs look nice on paper but we don’t have any experience building them. It would take a gigantic up front investment to work out the real world issues and commercialize a technology that has a lot of novel aspects like handling radioactive molten salt.
Meanwhile that same money would buy loads more power in solar/wind and batteries, which are proven technologies that are getting progressively cheaper.
An alternate timeline where we do MSRs in the 1950s and phase out coal by 1990 would have been possible but we didn’t do that and there are better alternatives now.
I have not seen any evidence that solar+wind will provide a proper base load of electricity, and it looks like MSR and its variants will give people the electricity they need.
While nuclear plants do pair well with storage (many pumped hydro storage stations were built to pair with nuclear plants), the idea that they cannot load follow is a myth. It is simply more economical for them to run at full load since fuel cost is a very small portion of nuclear operating expenses.
It will likely take a minimum of ten years to get a non light water reactor certified by the NRC. And that is very optimistic. Then you have to build the first of a kind plant which is always more expensive and takes longer. Then you have to get good at operating these new kinds of plants.
It's true that MSR and Breeder reactors have lots of potential benefits over traditional LWRs but the truth is, LWRs are more than good enough for right now and we literally can't build enough of them if even if we tried.
You wouldn't want to power all of human society off of LWRs simply because they only access ~5% of the energy in the fuel. But we're so far away from that being a constraint. Build LWRs today and keep developing Breeder/MSR tech.
Meanwhile that same money would buy loads more power in solar/wind and batteries, which are proven technologies that are getting progressively cheaper.
An alternate timeline where we do MSRs in the 1950s and phase out coal by 1990 would have been possible but we didn’t do that and there are better alternatives now.