What happens when the wind plants and solar plants aren't producing?
Peaking. Now do you have any other absurdly easy questions?
Covering average demand is ONLY covering average demand.
Yeah, it's not like variability has ever been a part of the grid before. Current grids have their own annoyances on the demand side, including daytime power consumption being much less than nighttime, summer and winter variations (sometimes major), etc - as well as also on the supply side, such as interlinks or plants suddenly dropping offline. It's not some sort of new ground.
The short summary of a high-renewables-penetration grid is:
1) Peaking plants (NG is a good choice).
2) Geographic smoothing (aka, while one front is leaving the US east coast, another is coming on the west; while there's a high stuck over one part of the country, a low is churning up winds elsewhere; also, midwest and east coast wind is strongest in the winter, while west coast wind is strongest in the summer)
3) Geographic timeshifting (aka, desert southwest sun is still shining when it's evening demand in NYC, the evening wind is blowing on the east coast during the morning rush on the west, etc)
(HVDC grid needed for #2 and #3 - est. 0,3 cents per kWh amortized cost for construction and maintenance, saving 1,1 cents per kWh in reduced generation hardware requirements)
4) Multiple source variability compensation (e.g., wind and solar tend to run opposite to each other - highs make low winds but lots of sun, and vice versa; winds are strongest at night, solar during the day)
5) Hydro uprating as storage. Optional storage additions = solar thermal, wind flywheel, battery (price is dropping fast), etc as needed/desired, but are not a fundamental requirement.
6) Demand shifting if needed (aka, power-hungry industries get favorable power rates if they're willing to occasionally shut off as needed; this is not a rare arrangement)
For the future, EVs also help, but are not required - insofar as they're mainly nighttime loads, steady draws, and easy targets for charge rate modulation (or even reversal). Nobody cares exactly when their vehicle takes power from the wall, so long as it has a full charge when they told it to be done by. The more flexible they let their car be, the cheaper they get their power for. But again, this sort of arrangement being wirespread is not a requirement - just a bonus.