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

[ViviAnimus]

No, at the status quo, mechanics of a fight do not matter greatly, because BLM will triumph. The fights where SMN wins over BLM is very slim and I would argue, simply do not exist anymore.
I have never seen a SMN do more than me on my BLM on Levi EX, but there it should be pretty close.
The charts comparing DPS are there for you to see, although they do not include savage coil. This is what happened. The magic classes were always kinda weak, but SMN was somewhat stronger. Then BLM got buffed to where it was quite easy to outperform SMN. Over time, more BLM got increasingly good (skill wise). You might have seen the numbers BLM like Lipton Icetea did before the buff. They were already beating SMN by quite a bit pre buff. The point that all jobs work in different ways, again, is true, but also complete nonsense. Extremely movement heavy fights would mean a BLM wouldn't be able to do anything at all. In every fight that involves movement, BLM can work its way around it. Actually, movement is worked around so well, that it almost isn't even a DPS loss anymore. Name me one mechanic that hugely favours SMN and how it makes the fight so that SMN will triumph over BLM.

I agree with your final statement. Hence my suggestions.