Can someone give me an in-depth explanation why Summoner is arguably the worst DPS?

[Sleigh]

Like endgame SMN's are doing almost the same damage in i130 as they were in i115 just because their party members are killing stuff too fast for DOTS to tick. When your party members effect you that much, there's a major issue.

BLM DPS gets affected negatively too by fast kill times, and is very much tied to party - Foe's, Selene, amount of Flares on adds, letting BLM sit still for mechanics, all a group effort for sure and a party can hurt a BLM just as bad as SMN. I think it's just that long-lasting DoTs on adds were the only saving grace a SMN had in T10, 12, or 13, so you'll notice that loss more in current gear.

[Judge_Xero]

BLM DPS gets affected negatively too by fast kill times, and is very much tied to party - Foe's, Selene, amount of Flares on adds, letting BLM sit still for mechanics, all a group effort for sure and a party can hurt a BLM just as bad as SMN. I think it's just that long-lasting DoTs on adds were the only saving grace a SMN had in T10, 12, or 13, so you'll notice that loss more in current gear.

Oh yeah there's no doubt that BLM DPS is effected by the party as well. But it's one thing to ask your Bard to play Foes or the SCH to use Selene, but to ask your party to stop killing things so your DOTS have time to tick....well /votekick

IMO I hope that's where SMN get addressed. Being able to swap targets for burst damage outside of being able to Bane is just too weak. 3 GCD to set up Dots in order to Fester something that your DOTs may only tick a couple times on is such a waste.
500-600 Potency where a BLM is 900-1200 Potency (hurray SMN Dots are ticking on another mob, now you might be up to 1000 Potency for that time frame)
Yet in a lot of Endgame you run into those instances where a mob needs to die fast or it does something terrible. Which SMN windup is just so slow and it's ST damage is just too low to have someone else swap targets instead, i.e. Melee.