That's part of what I'm getting at. The problem is the 5th chest is just that; a fifth chest. It would be nice if it's odds of dropping something worthwhile were modified by something players could do, like as you say - based on # of trash mob kills. That's somewhat in line with my time-estimation concept just done differently (you're losing time by killing the trash mobs, essentially wagering that you will be able to beat the rest of the run fast enough to make up for it). Anything other than yes/no 5th chest based on a static time limit would be a little more engaging I think
I get what you're saying, and as for the speedruns you may be right. I'm just saying that if they're dead set on the method of increasing drop rates being speed runs, why not make the system a little more engaging and exciting than what it currently is.
It could even be something like how in some action games (Devil May Cry comes to mind) your "grade" is based on multiple factors, such as kills and run speed (could even give a bonus for class variance), rather than just one. Personally, I would prefer the challenge being beating the dungeon and only being able to do so on a longer timer as you say (and with an accordingly higher drop rate). There's other way's of doing it but it just seems how it's being done right now is boring and discouraging. I'm fine with spamming speed-runs personally but I would definitely prefer something that's actually exciting/fun.
You're probably right with the ifrit examples, and that because sear is unavoidable and plumes are avoidable is what causes the preference for ranged DD's. I just hope they don't tailor garuda for melees just because ifrit ended up being tailored for ranged DD's. Again, I'd just like to see a more interesting system that doesn't punish class stacking but makes it worthwhile to split the classes. For example, in AV, BLM is great for most of the run, and then MNK at the end. It would be nice to have at least some variety/reason to use multiple classes/jobs. What I was suggesting is essentially a dumbed down version of Skillchains/Magic Bursts. I definitely like combo's, but I'm just saying theres plenty of things that could be done to encourage class variance in a single run, rather than class variance for different runs.