SMN actually does run out of MP around the same time as TP users naturally run out, in fact SMN can last longer than most of them (around 4m20s if played normally). The problem is, most classes run out around then or sooner, and since SMN is on a different resource pool, it is not going to get help in the same way. The current BRD resource system, where TP song lowers DPS, MP song lowers DPS, and Foe's doesn't lower DPS, and it's all tied in the same MP pool for BRD, means MP isn't worthwhile to just sing for a SMN.
Buff SMN's Ruin filler to do more damage and/or somehow either cost no MP or actually recover MP, or let pets at least gain the effects of SS.
80%+ of SMN's damage (Aetherflow stacks tied to 60s CD, DoTs, pet attacks and CDs) is completely unrelated to SS in the current system, meaning you could have 0 SS or 200 SS and it won't actually boost a massive portion. I don't think SS needs to be useful exactly on SMN but in a gear system where it's on most of their pieces, something should change somewhere, and I'm pretty sure they're not going to shift all the itemizations around mid-patch and receive backlash from the BLM community.
Enough that BLM, the least mobile DPS in the game by default (which can be gotten around, but not 100% of the time), beats SMN, the second most mobile DPS, in the most movement heavy turn in the game, T11.
Honestly it's not about how far SMN does below the top, it's how badly it gets obliterated by BLM (who is also explicitly in the AoE, magic damage, caster tools role and typically only one caster is brought to fights to maximize DPS) in a dummy and raid scenario. BLM doesn't need nerfs, SMN just needs to be brought up to speed, or changed in a way that justifies their presence.
This is definitely true, BLM has to have higher raw output to compete with SMN in mechanics heavy fights, but the gap is simply too wide presently, not counting MP issues (where the true gap starts to show itself). Even 10-20 DPS more consistent DPS in i130 would go a very long way to balancing the two classes.