First, you can't just have Ruin under one umbrella, they all do different things. Ruin is designed as SMN's go to DPS, Ruin II is an instant spell, Ruin III is designed to be used during Dreadwrym Trance and Ruin IV which does share a slot with Ruin/RuinIII has a chance to proc. Combining them to one skill would only serve to nerf SMN greatly since they would no longer have access to their instant cast Ruin Spell.

If you want to know why SMN is hurting so much, read the abilities that BLM and RDM use, ask yourself, how do they synergize with the job, then look at SMN and ask yourself, how do they synergize with the job. When you notice the huge flaw in SMN's current Abilities, you'll understand why they have a hard time compared to BLM and RDM which have great abilities that flow well into their playstyle.