Why does the law of diminishing returns apply? A lot of things aren’t diminishing returns.
Edit: to clarify, lasers will have some maximum efficiency that is less than 100% and approaching that maximum is subject to diminishing returns.
I wouldn't bet on no breakthroughs happening in laser efficiency, but more efficient lasers doesn't look like it will be enough to get to net energy given other inefficiencies in the system.
But that’s not what we’re talking about. This is a physical process which is known to be exothermic for the energy ranges we care about.
As another example, raising the temperature of a flammable material 1 degree from room temperature will probably not light. Ditto with 2 degrees. But eventually, if you raise the temperature high enough, you’ll get more energy out than you put in. That’s the type of process we’re talking about now.