At first impression, I honestly agreed with everything. But the more I thought about it, I actually have to respectfully disagree with everything in this take (yes, even point two).
While Breaker Lance does debuff Atk which RS scales off of, it also debuffs Def, diminishing the returns of moonbow and the whole point of even running it.
RS isn’t really that niche as you’re making it out to be. The generalized rule and relationship of RS and moonbow is that RS will have the same scaling as moonbow if the enemy’s Atk is 150% of its Def/Res. If the enemy is a dragon/beast, it only needs to be 120%. This isn’t exactly a hard to find case to encounter in AR and SD.
tl;dr: @Tophez RS will always be better than moonbow if enemy’s Atk >= 150% Def. Just ask yourself how common you see that case based on the game modes you’re personally invested in to make that call if it’s worth it.