The main purpose of these changes are to bring it closer in line with PLD for the purpose of main tanking and WAR for the purpose of off tanking, while still giving all tank jobs their own unique advantages. DRK's advantages currently are their DPS and Magic Damage Mitigation when in the MT role. However, given the current state of content, we're able to see it's shortcomings rather easily.
In the Main Tank role, PLD currently has an edge with it's supreme defensive and healing toolkit. This shouldn't be changed because it finally gives PLD the uniqueness that it deserves. DRK's current DPS in this role does not make up for the difference in mitigation, unless the fight is one where the enemy does not hit very hard for any extended durations. Reprisal is the biggest culprit to DRK's shortcomings and needs to be adjusted first. I don't believe the changes I've suggested will eliminate PLD's usefulness. PLD will still maintain an impressive array of defensive CDs as well as party utility via Clemency, Divine Veil, Cover, and Stoneskin. What DRK will bring to the table for sacrificing those advantages are: higher DPS as MT and powerful self-healing via draining attacks. WAR's contention is and will be their frequent use of Inner Beast for both DPS and mitigation, along with their strong defensive and self-healing toolkit.
In the Off-Tank role, WAR currently has an unprecedented advantage with Storm's Path and the highest DPS, supplemented by their increased stamina via Equilibrium. PLD's party support provides an alternative route for parties with Clemency, Divine Veil, etc., allowing for more variation in their approach to difficult content. DRK suffers from lower DPS than WAR and their only party utility currently is overwritten by MNK's Dragon Kick. The suggested changes will slightly boost their DPS, while also alleviating their TP issues, making their damage output slightly higher than WAR's. I believe this trade to be a fair one, given the strength that Storm's Path brings to a raid.