It's true that to get to 100%, or to start a new grid, you need to have something to fill in the gaps. Here there's a wide array of options, of which the most deployed at the moment are hydro and biomass AFAIK. But there are other proposals.
In the past it mostly didn't make sense to store electricity, not when you are paying for coal or gas and could as well store that. So many of the latest proposals are not fully developed and operating at a massive scale yet.
Hydro as a natural resource is not a perfect fit to use in case like this as natural lakes can only take so much changes in water levels, and in many cases you have chains of hydro plants and lakes which mean dumping a lot of water in one place means you have to dump a lot of it further down without causing flooding. Hydro plants must also manage how much water they have available. Swedish power prices is notorious effected by how much water winter/fall periods produce.
1) Nuclear, with the high building cost and externalization of spent fuel.
2) Renewable, with cheap building costs and externalization of variable production rate which is solved by burning fossil fuels like natural gas or coal.
If we look beyond current grids and look into the future there is solutions to those externalizations. Renewable could use batteries rather than burning fossil fuels, but the current best bet is to build enough hydro powerplants and dams to fully support the energy grid during low production periods, and pumping back the water during high production. This will cost a lot of land and put a lot of greenhouse gases into the air, in particular because putting a lot of land under water tend to create a lot of methane. The cement for the dams are just like the cement in nuclear plants a major contributor to greenhouse gas production.
Personally, I just would declare that the age of burning fossil fuel to manage the energy grid has come and passed. By 2020 we should not build any new natural gas power plants. by 2030 we should have decommissioned 90% of all fossil burning plants, and by 2040 a complete ban of burning fossil fuel for electricity for any scale larger than a hospitals backup generator. With those decision in place I trust that investors will figure out if nuclear is profitable or not, and I personally do not care. For the climate sake my only wish is that we stop burning fossil fuels.