France's total electricity production from nuclear is expected to rise, actually: the government, after abandoning the plan to have nuclear covering just 50 % of its electricity mix, removed the cap on the total nuclear power installed. In addition to that, almost all the existing reactors got their life extended (so up to 50 years of operating life), and the final aim is to reach 60 years. 6 new reactors have been approved, and another 8 could get approved. Basically, the whole President Holland's plan from 2014 on reducing nuclear has been dismantled.

Denmark's renewables are highly subsidized, construction is quick (which means short-term loans), so it should not impact so much on energy prices. Finland has built the first-of-a-kind highly delayed EPR 3 reactor Olkiluoto 3, and the electricity prices felt sharply nonetheless.

I do not see the reason why decommissioning costs should make electricity more expensive in the future, because the owner has to pay and prove that the decommissioning will be done even if the company goes bankrupt (by allocating funds in advance or by providing an insurance).

This is not to say that a solution is better than the other: it is always a matter of finding a good energy mix for a network. France is investing in energy efficiency and renewables as well.