Dielectric mirrors can reflect 99.9% of light in a wide spectrum, and 99.999% for narrow spectrum and limited angles. So even if we take the 99.9% one, your 50

Dielectric mirrors can reflect 99.9% of light in a wide spectrum, and 99.999% for narrow spectrum and limited angles. So even if we take the 99.9% one, your 50kW laser will only heat it with 50W of power, which is nothing compared to the heating an ICMB (or its warhead) has to withstand.