Low electricity costs are a problem for all electricity plans, not only nuclear.
Renewables are even more susceptible to this because the weather affects all renewable plants of the same type similarly across wide geographic areas, so there will be times when they all generate more than is needed and nothing can be done about it. What's worse, the output is unpredictable, so there's little opportunity for some business to base its operation on renewable electricity generation patterns.
At least the weekend demand drop is predictable, so businesses can use this predictable opportunity to reduce their costs and thus reduce the impact the weekends have on the profitability of nuclear and similar dispatchable plants.
Renewables are even more susceptible to this because the weather affects all renewable plants of the same type similarly across wide geographic areas, so there will be times when they all generate more than is needed and nothing can be done about it. What's worse, the output is unpredictable, so there's little opportunity for some business to base its operation on renewable electricity generation patterns.
At least the weekend demand drop is predictable, so businesses can use this predictable opportunity to reduce their costs and thus reduce the impact the weekends have on the profitability of nuclear and similar dispatchable plants.