Non-random damage contributes to the green DPS paradigm, but isn't the ultimate creator of it. Downtime is. Downtime will always exist. It's a natural consequence of designing a game that allows a wide range of skill levels to complete most content. Random damage will slightly reduce downtime. Adding support will reduce downtime. Short of doing something utterly insane like disabling healer damage spells in instanced content, it's not possible to eliminate it. Or, based on how low Squeenix wants to maintain the skill floor, reduce by any significant margin.

I've said it a hundred times and I'll say it a thousand more: This community doesn't actually want a huge increase in outgoing damage. The proportion of people who think dungeons or story trials are challenging prevent it.