Yeah, but that requires quite a lot of time. D-rank qi isn't a big deal on our scale - it's much more impactful in group engagements with lower level people on the field.
The dps here is high enough that I wouldn't expect things to drag out long enough for it to matter, and we'd be better off casting HR or Eagle for some more dps.
Eh...It's a lot of time if you're expecting it to drain him out alone. Keep in mind that he has to
spend qi to fight us. Also keep in mind that Lunatic Whirl and Elegy and Refrain are also draining him. Also keep in mind that it'll take a fair amount of hits to down him, especially with the defensive approach. If we aren't in rushdown, we only lose two Rank C attacks (neither of which is guaranteed to hit) spending our action on it, and it gives us the extra qi to use an extra C rank tech every 2 turns. Which could be USR, an offensive perfect, or Toast, for example.
And frankly, if we aren't in rushdown, I'm not sure we'll win by damage. I mean, he has to be activating at least a C rank tech every round to keep up. (Probably 2 techs or a B rank tech, in practice.) He probably has A rank qi. With it and Elegy up, and HR's drain probably active for at least part of the fight, that's probably 10 rounds fighting all out. Add the threat of Lunatic Whirl and Isolation, and it drops even lower. We can play defensive and trade perfects till he runs out.
And while our damage is a faster clock in theory, that's because we can continue investing more actions to do more damage, and doesn't count the fact that damage is much easier to defend against. If we just left it up to our DoTs, we'd probably end up with a similar ~10 turn clock. And as for defenses... He has to burn Qi to do it, of course - but between forcing him to burn C rank qi to perfect an attack, and forcing him to burn D rank qi every turn for the rest of the fight, I know which one I'd pick.
It's different in rushdown, of course. Rushdown is based on pouring all our actions into damage in the hope of winning quickly. But in a defensive strategy, the techs that just have to be used once and then win for you if you keep up long enough are more efficient than one more hit. If you were comparing it to a DoT like PWC, it might be less efficient, but single hits are much less worthwhile.
It's still possible we'll win by damage, maybe even probable. But that means he decided to spend less qi on defense. Qi Drain's ability to win the fight is secondary to its ability to keep them from doing as much stuff.