I'm sorry but the lifespan of the power plant is irrelevant.
LCoE is measured in dollars per MWh. If your powerplant is projected to go from $55/MWh to $110/MWh no amount of lifespan doubling is going to change that.
It's not irrelevant. The cost to construct the nuclear power plant is very high, and is the main part of the price, and if it's amortised over 20 years, the cost of MWh produced is higher than if it's amortised over a more realistic 50 or 70 years.
Locking up capital for 70 to 80 years? Do you know about the time value of money? [1] What you are proposing is FDR and Churchill sitting at the Casablanca Conference at the height of the second world war in 1943 [2] planning out economic investments that will be returned in 2023. Does that sound reasonable?
Likewise, a renewable plant that makes the money back in ~25 years allows the investor to reconsider the investment decision and better optimize the capital allocation two times while the ones building nuclear are still paying for the nuclear plant. The renewable owner could after 50 years decide to invest in nuclear. I wonder which is more efficient?