You spin and cool your weapons, and make them as reflective as practical and this system won't do any damage
I've never seen anyone agree with this assessment who has professed knowledge of the subject. The issue is that lasers still dump a lot of energy, which starts to mar the mirror really incredibly fast. 2.5kW of energy is like applying a soldering iron tip to the mirror, which rapidly (1s) causes it to mar and become useless. Not only that, atmospheric pollutants, water droplets, grease-- anything-- will make that part of the mirror useless. Applying mirrors to your turrets? Good luck keeping them clean with all of the gunsmoke coming out of them.
Meanwhile the defender has the burden of carrying all of this extra weight (not really feasible with aircraft, seacraft, or missiles) and trying desperately to keep it clean in battlefield conditions, plus all of the added expense for an armor that is, at best, marginally useful.
If your enemy is having to adopt incredibly expensive, incredibly hard to maintain, and marginally effective countermeasures to your weapon, then your weapon is doing its job. It really doesnt matter whether you blow up 10 of the enemy's ships, or whether their countermeasures have meant they could build 10 fewer ships; the outcome is the same.