Regarding stability, this physical inertia is also present in rotating wind turbines. But I guess exploiting this at a meaningful scale would recquire a level of interconnectivity which boils down to the same issue of cost.

I am however optimistic about grid-scale storage. There is a long term trend of rapidly dropping battery prices, and with recent developments in sodium ion batteries there is no fundamental reason this won´t continue. Another enabler could be advancements in lifespan. This could allow storage being installed inside or near wind and PV, cutting down on space and installation costs. Even then however, grid improvements would be needed.

Some problems still need to be solved indeed, but in my (mostly uneducated) opinion, they seem easier than achieving economically viable fusion. But they do still require large investments in R&D and manufacturing capability.