I would disagree. Motivation is only half the factor. If players aren't enticed by a 15-17 minute dungeon versus a 25-30 minute dungeon then they probably aren't going to pay attention the ranking system if it doesn't amount to anything beyond a pat on the back.

The game needs to put players in a position where they have to improve - Even just relative to the content they are doing. If players aren't in that position then a lot of them won't improve. Players that aren't improving under the current system aren't going to start improving because they got a pat on the back from a score screen.

Instead of a ranking system, it would probably be better to have duty-specific goals/achievements, e.g., "Kill Merciless Matriarch: Rukshs Dheem in Anamnesis Anyder without spawning an avoidable Queen's Harpooner" - Or you can have one on the flipside for "Kill Merciless Matriarch: Rukshs Dheem in Anamnesis Anyder whilst spawning all possible Queen's Harpooner (dependent on kill time)" - Then you are encouraging people to clear the duties in specific or creative ways, versus a... "You completed Tower of Zot in 15 minutes (sync'd) - Rank Achieved: S Rank"