"Summoners damage starts off high then declines over time because of the diminishing returns built into a DoT dps"...
What..!
Your DoTs tick at the same rate they do at the start of a fight...
If you even play as a SMN, then you'll realise that the reason why your dps "diminishes" after a certain period, is because of (1) a phase transition where DoTs haven't been applied for a boss 'jump', or DoTs do not tick during the 'jump', (2) the mob(s) that are available die too quickly for you to full DoT AND get a reasonable amount of ticks on, and (3) the most important, FESTER -- w/o your nuke (on-hand), you're relying solely on all of your DoT ticks every 3 seconds + Ruin I spam/ Ruin II GCD filler/ pet buffs/ pet AoE to give some spike to your dps.
Both BRD and SMN do high initial burst due to unloading CD's. Whereas the BRD starts intially very high, and then drops immediately to a much lesser plateau with occasional small spikes of dps (due to CD's/ proc luck).
SMN on the other hand starts high, and continues to remain not too far below the initial pull dps until their Aetherflow stacks are burnt out, and they can no longer Fester as and when they wish, because Aetherflow is on CD. Fester is a much higher nuke/ spike than anything a BRD has to offer.
Compare that graphically to melee/ BLM, and you can quite clearly see that the problem is the inefficiency of Aethflow availablity to SMN, in order to remain competitive, whilst at the same time it is also the lynch pin of mp regen/ returns, because Energy Drain (with it's current potency) is not a viable alternate to Fester because of its poor mp returns (e.g. all 3 Aetherflow stacks are need to get enough mp back after death in order to be able to get your pet back up, and have adequate mp left for DoT'ing there after).