It can't be true that keeping a nuclear power plant running is more costly than replacing it with a massive capital expenditure. Can it?

The only possible way is if the alternative is massively funded by debt, and the "cost" is based on historically low interest rates and a repayment timeframe in the decades.

Even then, given the ongoing cost of nuclear is relatively small, how is it possible that the capital expenditure of solar + natural gas is cheaper than the running cots of nuclear? Can someone show me that maths?