Literally everything has diminishing returns because nothing is infinite.
Edit: to clarify, lasers will have some maximum efficiency that is less than 100% and approaching that maximum is subject to 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.
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.
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.