Subpar? Did you update spreadsheet on MNK for Taeon and Impetus 119 emp body or parse any Koga vs Glanz lately? Koga is very beatable nowadays and tho I admit I'm mainly stuck on mage [geo/rdm/sch/blm] too often, NIN was right along side MNK on spreads. I'm talking 1500 Set DPS for both MNK and NIN while SAM was around 1100 with same exact buffs. Which should mean something even if you don't use or believe in spreadsheets. Acro didn't significantly boost 2handers, including SAM itself, as much as Taeon has done for 1handers. Until SE offers something beyond "Oh boy, you have found my THUNDER weakness! Please help me help you kill me!!" as it stands, the best DD in game as of atm is actually BLM. Which would be interesting to see how BLM would parse now vs older content like Delve.
The only thing that separates SAM and MNK dps from NIN is that they can flat out DD solo while NIN still heavily relies on other DD in pt to pull/hold hate so NIN can then utilize Innin /ja 30% crit rate boost. Other than that DPS is altered by other intangible variables making one DD > the other depending on target.
I think you are likely doing something wrong if you have Monk anywhere near Sam or Blu these days. Your DPS numbers are extremely low for what you usually get spreadsheeting, I'm not sure what buffs/targets you are using but generally speaking it takes a lot to get down into the 1100 DPS range. Using pretty normal buffs on the harder targets the spreadsheets allow (Tojil generally) you should see Koga Sam in the 2k+ range at least with Monk around the 1800 range with all the other subpar DPS. Taeon was just as much a buff to Blu as it was to Mnk and they also got Job Trait, CDC, and all sorts of cool stuff lately and they are far and beyond better than current Monk. Sam has better WSes, TP gain, SC ability, just about everything, in a game that is entirely based around WS and SC damage a job based on white damage does not do well. Monk isn't even in the top 5 of DDs even with Taeon.
You have to consider that Monk has barely been touched balance wise while most jobs have seen huge changes such as the Rudras buff, the addition of Daken, Gifts that absolutely break jobs (Blu), new spells, revamped abilities, ect. Just adding a new set of gear that benefited a large group of jobs doesn't do much compared to that.