My guess would be this: the cost of a reactor dominates the cost of electricity for fission, so making a reactor last longer reduces the time-averaged cost. Typ

My guess would be this: the cost of a reactor dominates the cost of electricity for fission, so making a reactor last longer reduces the time-averaged cost. Typically, reactors are expected to pay for themselves within 10-20 years. So, a reactor that self-destructs in 5 years needs to cost 2-4x less. It's not clear that MSRs have that kind of up-front cost advantage.