I'm not entirely sure what you mean by 90%. 90% of what?
You can't just have 90% of power generated by renewables and 10% by fossil fuels because there are times when you get 0% from renewables and so your fossil fuel plants need to provide 100% of the power. So you spent all that money on renewable infrastructure and didn't even get the benefit of shuttering your fossil fuel plants.
You can't just have 90% of power generated by renewables and 10% by fossil fuels because there are times when you get 0% from renewables and so your fossil fuel plants need to provide 100% of the power. So you spent all that money on renewable infrastructure and didn't even get the benefit of shuttering your fossil fuel plants.