Land costs are indeed an immense issue. The exact figure of how much land needs to be covered in solar panels varies depending on inclination with the sun and weather.Its not too bad for countries like Australia that have reliable sunlight and plenty of land. But much of the energy consumption occurs closer to the poles, and with more frequent obscuration of the sun from weather. And these countries are typically much more population dense. Dedicating several percentages of the landmass of a dense country is indeed a substantial cost. And this isn't even getting into the require immense amounts of energy storage. We have 5 minutes worth of hydroelectric storage relative to our average 11.5 TWh daily electricity consumption. But hydroelectric is geographically dependent and hard to scale. We only have 10 seconds worth of battery storage, about 1 Gigawatt hour.
It's incorrect to say that countries never go all in on one energy source. Norway generates almost all of it's electricity from hydroelectricity. Iceland generates the overwhelming majority from geothermal. And France generates the lion's share of it's electricity from nuclear power. The last of these three is geographically independent.
Sure, Norway and Iceland probably don't have to build a single nuclear plant. Nor would some states like Vermont and Washington that have extensive hydroelectric generation. But other geographies can only make do with fossil fuels. Intermittent sources can mitigate this, but we'll always need a solution to fill in the duck curve until we either make a breakthrough in energy storage or some other carbon free form of energy. But we already have another form of carbon free energy, and one that is already working for other countries.