If you mean renewables by that, it's hardly hypothetical or unproven. I'm in Australia and south Australia and Tasmania (two of our states) have fully renewable grids, Tasmania for the past 7 years. South Australia does still occasionally pull from an interconnect but most of the time they're exporting a bunch of power.
Renewables with storage are cheaper and faster to build than nuclear and that's from real world costs. Nuclear would be fine if it wasn't so stupidly expensive.