You're paying the capital and maintenance costs of the fossil plants whether you have solar or not. They are not "driving up the price" because they're not new.
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!
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).