There will always be downtime. The time spent in an encounter can be split into two major categories. The first category:
1. Time spent on healing that's required by the encounter.The second category:
2a. Time spent on healing to recover from people making mistakes.It is certainly possible to change job design and encounter design so that more time is spent on (1), required healing. And that would certainly take away from time that could be spent on (2b), DPS. However, it would also take away from time that could be spent on (2a), recovering from mistakes.
2b. Time spent on dealing damage.
In roulette-level, story-mode content, there absolutely must be time available to spend on (2a), recovering from mistakes. Expecting perfect play at that level is exactly what will wall off players. At the same time, there will be parties that make fewer mistakes, or have better gear, so mistakes are more easily recovered from, meaning that we'll be able to shift time from (2a) to (2b), dealing damage.
