Cooldowns are a thing on buffs anyway. Yeah, a long one can be an issue, but buffs are generally viewed higher than debuffs considering they're less likely to get canceled
While true on the combo thing, again, high debuff resist can just give a middle finger to that. So then what? This more or less comes to Ama having two skills into one, which still makes her better in that sense if it wasn't for the fact that all of yamas debuffs are in one skill, which more or less shuffles it around between the two
As for damage cap, you kinda contradicted yourself and proved my point at the same time. Can't hit 1 mil without doing 3.4 mil cause the cap is 700k, even though it's clearly doing higher than the cap? What's really happening is the same as a mechanic on Elsword known as normalization, meaning when you hit a certain point, you get less payoff by increasing it... which a lot of things in this game also does. But since "doing as much damage as possible" is the main focus, the average damage of both skills really needs to be looked at. Also, I did say it can hit 7 digits and you explained a way how...
Now granted(and i did say this in the last comment) Ama is better cause she has a wider range of effects (despite being defensive in a DAMAGE meta), but at the same time yama isn't weak either, you're just refusing to give her more credit. Plus you're gonna have kami with buffs as well if you got for the "best fire kami" so until someone rolls around with a better kit including the same or stronger fire atk buff, that's a bit less damage getting pushed out.
Plus if you got a team to hit the debuff cap anyway, how much past it you can get would still toss yama into the picture. Why not go for another buff to increase damage output (usually on bursts which fire is really good at) when both ama and yama can help hit the debuff cap?
Hell, I've even been told multiple times before that getting ama on a ticket was redundant considering I had yama. Only real reason I got ama is due to defense in fights where I can't debuff
Gotta keep in mind that it can depend on the player's situation