Up to a point because transformation and transmission losses are a cost killer. Also that still does not help the fact that solar needs to be backed up so you have not negated the other facility costs.
No doubt that minimal wind can be pretty reliable in some places.
As for solar, the sun rises and sets all places and we also have clouds in most places. Solar in dry deserts addresses the clouds but unless you are Joshua you have to face the night.
While it is true that solar and wind resources have difficulties associated with them that spinning resources do not, the difficulties are manageable. ERCOT, the grid operator in Texas monitors weather forecasts and climate data, and they can predict pretty well when renewable resources will be available, or not. And of course sunrise and sunset times are predictable. Coal and nuke generation cannot be switched off and on quickly, but natural gas generators can. Also, many utility scale PV inverter companies are starting to engineer soft start and stop functionality as well as power factor correction and power limiting capability into their products.
There are challenges associated with the incorporation of renewable/sustainable energy sources into grid distribution, certainly, but they are not insurmountable.
