You have to imagine, before opto, EVERYTHING in neuroscience was observational, except for the only real circuit intervention we had - ablation of an entire region without any specificity, especially with regards to passing axons.
Now, I literally expect any near paper that even touches behavior to have some sort of validation with opto, or it's harder for me to believe.
Maybe there's a bit of an over reliance on it in my above comment, but if you have a better causal tool to suggest, be my guest.