You don't have to theorize: you need substantial amounts of spare capacity because with solar, for example, there will be night and winter, the costs of maintaining this capacity have been substantial in practice and have driven up energy costs everywhere (the confusion here is about what the article is claiming to say vs what is actually happening...the article constructs a model to show that energy prices would be higher, the problem is that the model is useless).
And in the real world renewables will never go to zero so you can reduce that cost when you build a lot of renewables, even accounting for the worst case winter.
Am I saying that overbuilding solar from a financial perspective never happens? No. But I am saying that if money is your priority, it's straightforward to plan and build renewables in a way that strictly saves you money. Even though you'll sometimes be paying people to do nothing!
No, it doesn't have to increase the cost.
If you have a town powered by gas, the cost of maintaining and staffing the gas plant is locked in.
But most of the cost of that gas plant is the fuel.
If the total cost per kWh of a solar or solar+battery installation is lower than the fuel cost of the gas plant, then you build it. It saves you money even though you're paying the gas operators to do nothing part of the time.
If it's not cheaper than fuel, you don't build it. No harm no foul.
Follow that strategy and you'll end up with lots of renewables without wasting a penny.
Though honestly some idle gas plants don't cost that much. How many kilowatts do you need? 4? Okay, the fixed costs for 4 kilowatts of combined cycle gas power are $50 per year. That's all it takes to have backup production for the entire grid, even with no base load plants anywhere.