Diminishing returns usually applies if you assume there are no major breakthroughs. Can we assume that there won't be any major breakthroughs in this field?
Diminishing returns describes a trend. A breakthrough describes a single data point that bucks the trend. I'm not sure these are mutually exclusive, as after any breakthrough the diminishing returns trend is reestablished.
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.
