Well, there's that too yes but I think that's not the main thing people mean in this context. The issue is that in most parts of the world a 100% renewable grid can't satisfy demand on a still night, and storage tech isn't there to ride you through - but even if it was, long stretches of cloudy still days are easily possible. Also the grid is very hard to restart once it has a big enough outage. Even once power is available at all the stations again, a black start is no joke and most countries have never been through one ever.
Why would anybody need a 100% renewable grid? Renewable grids will be 150% grids, that provide 50% more load than you normally need and 100% for 98% of the time. The remaining 2% of time where renewables can't deliver (windless/sunless winter days) will be covered by gas peaking power plants.
The goal is to avoid CO2 emissions at reasonable economic prices.
You can't build 2x the generation capacity (which is what you're suggesting) at economic prices anyone would today consider reasonable. Remember you have to be able to cover 100% of demand from non-renewable sources. It's not just the gas plants, it's the pipeline network, the gas wells, the LNG import terminals, the ships, the whole shebang. It's extremely expensive to have it available but not actually use it.
