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

[Aikaal]

I love SMN, I still play it from time to time and it still has my heart as my "most" favorite class but I can easily accept that they are weak.

That,s exactly what I want to mention. SMN is flawed right now, you're not forced to stop playing it.

with a base amount of 214 piety, our conversion rate should be way higher than blm to the point that without crafted gear or food smn should be sitting around 4k mana pool.

I have infinite mana but you have no idea how the BLM rotation works. This is a territory you shouldn't enter I have a base of 245 Piety because of my BLM Soul. A lot of people give few Piety points as a BLM for their rotation. Some of them go crazy and meld to have 316 Piety to get an extra Fire 1 in their rotation. I want to keep my large mana pool.

The mana pool of BLM has nothing to do with Summoners.