I'm so glad you ended with the word "playstyle" because it speaks volumes.
Regardless of whether SE considers optimum rotations when designing jobs, there is an optimum way to play them, and many people myself included have tried to point out ways and techniques to make the job live up to its full potential in its current iteration. Playing better will save you time, save people that play with you time, grant increased satisfaction from playing in general, and is just straight up more fun. DRK is an incredibly powerful job when played well. Hell its been world first MT in every raid tier since it was introduced.
My personal opinion is that if you aren't playing the job to its full potential (and many of the suggestions made lately seem to indicate such) in its current iteration, you shouldn't be calling for a new/different one. Some of the ideas put forth have been terrifying, and it personally grills my cheese when someone tries to get SE to redesign a job I can play very well, as in my mind that reads "The job should play like this and all the people that can play it well as-is should have to relearn the job they way I want it." DRK isn't a perfect, flawless job, but most of what it needs are very very small tweaks, and most of them are utility-based or very, very small defensive tweaks. Its DPS is PERFECTLY FINE where it is. DRKs that are trailing behind their WAR buddies by hundreds of DPS are doing something wrong. Assuming you both have full uptime a DRK should only be a very small amount behind WAR or matching the WAR depending on the fight. The gap between WAR and DRK DPS is pretty much slashing-debuff-sized. WAR is also a very popular job and is frequently OTing where DRK is MTing, and is often given preferential treatment in raids, which accounts for bigger differences you may see on FFlogs.
Many of us are arguing against you and other posters because your suggestions completely break DRK (making it use HP instead of MP for a resource) or make it completely destroy the other two tanks (bringing the Sole Survivor buff to PVE or boosting Darkside to 30(!)% increase). We're trying to dissuade people from making suggestions that would only lead to months and months of the other two tanks demanding buffs in return since they'd be left in the proverbial dust, or months and months of "DRK: They Broke My Job" threads a la BRD at 3.0's launch.
DRK doesn't lose anything other than Blood Weapon by going into Grit. You still get to use all of your highest potency attacks, and if you're in Grit its assumed you're tanking something, which means you get Low Blow and Reprisal Procs as well.
Statements like yours irk me because hundreds, maybe thousands of us have done the math ourselves (even if we suck at math, like myself) or checked the math of others out of intellectual curiosity to know things like what you're calling into question and asking us to prove again, just for you, because you decided to remain ignorant.
Honestly with SSS now there's zero excuse for not having mathed out this kinda stuff yourself. You can also take a sample size of a given tank on FFLogs and look at things like the % of DPS added with Low Blow and Reprisal for a given group of DRKs (and Blood Weapon which I believe was mathed out to be a 6% increase not accounting for increased mana/Souleater), the % increase of DPS added with Shield Swipe and SwO (looking at AA damage by a factor of 1/3rd for the PLD during SwO uptime, iirc mathed out to be an 11-13% increase) for PLDs, and the % of DPS added with Fell Cleave for WAR and then add those numbers to the following:
Defiance is .75*1.2= .9
Grit is .8*1.15= .92
ShO is .85*(1+(.3/3=1.1)= .935
Deliverance/Maim is 1.2*1.05=1.26
Blood Weapon/Darkside is 1.15*1.06=1.219
SwO/FoF is 1.1*1.1 is 1.21
WAR - 1.26-.9= .36
DRK - 1.219-.92=.299
PLD - 1.21-.935=.275
These numbers are probably off but their proportions relative to eachother between the tanks are more or less correct.
Now add the % increase of Reprisal and Low Blow procs to the .92 from the DRK equation, the % increase of Shield Swipe procs to the .935 from the PLD equation, and the % increase of Abandon abilities to the 1.26 from the WAR equation and you see that DRK has an extremely small gap between Gritless and Grit, and while PLD's is even smaller, PLD can't match WAR with a slashing debuff, but DRK can in a lot of cases and Reprisal+Low Blow are collectively much more powerful the Shield Swipe. WAR is the largest, but it also has Unchained. You could math this out to a 1.055% overall increase if kept on cooldown but we all know WARs don't just sit in Defiance and Unchained every time on recast so its more difficult to account for.
As I said, I'm not a math wiz or a Dervy or anything but even these rough, napkin math numbers show that DRK's MT DPS is the highest, then WAR's, then PLD's (PLD comes close to WAR if given a slashing debuff). In OT DPS, WAR is top, followed very closely by DRK if given a slashing debuff, then PLD.