Yeah I think you would still want to have some Dexterity as a Riposte Rogue, assuming you're going for a damage role and not one that is purely tanking. Even though Riposte is independent of your recovery bar, it doesn't quite supplant the need for Dexterity on a striker Rogue for me, at least for the purpose of making non-Riposte, normal attacks which I think would remain the main form of damage. Like I mentioned in an earlier post, Riposte is just too variable (chance-based, relies on the presence of melee enemies, their attack rate, etc.) to be a consistent form of damage across all the different combat encounters you come across in the game. Just as an extreme example, if you were to make a Riposte Rogue with multiple sources of Riposte-like gear such as Tuotilo's Palm and Champion's Helm who dumped Dexterity and ignored standard attacking but pumped Deflection high enough to make enemies reliably miss their attacks, I think you would be disappointed by their damage output once you find yourself in situations like encounters that are light on melee combatants. I think it's probably better to treat Riposte as incidental, bonus damage rather than as a principal means of damage if dealing damage is an important element of your build.
But if you were to build a hypothetical Riposte Rogue, for subclass I think the Trickster makes sense as a baseline for the +30 Deflection from Mirrored Image. Then you multiclass that with a class that boosts Defense (even though you might think of Deflection as a subset of Defense, they are technically different stats and would stack if you have one active ability that states it boosts Defenses specifically and combine it with another active that says Deflection). So you could look at multiclassing Trickster with a Paladin for their Faith and Conviction/Deep Faith passive defense, Fighter for their Vigorous Defense, or Cipher for Borrowed Instinct (and Psychovampiric Shield as a further stackable bonus due to being classified as an Inspiration). Lowering enemy accuracy would help with making them more likely to miss and trigger Riposte, so abilities like Paladn's Glorious Beacon, Cipher's Eyestrike, and Trickster's own Persistent Distraction and Blinding Strike would have extra synergy. And of course, you need sufficient accuracy yourself to land the Riposte attack so abilities like Fighter's Disciplined Barrage and Cipher's Borrowed Instinct could be pertinent.
Here I go breaking my promise not to bring up WotEP any more, but the calculus for all of the above changes for me if wielding Whispers of the Endless Paths. By simply having a 100% chance to proc an attack roll from misses, I'd feel more comfortable maybe sacrificing some Dexterity and relying on Offensive Parry more as a legitimate damage source. While it would have the same issues as Riposte in those non-deal situations, it would at least have better consistency in the situations where counterattacking is applicable. The Trickster/Soul Blade combo from the OP in particular is nice because Offensive Parry would be passively generating Focus which could then be used for delivering Soul Annihilation in an AoE for burst damage to make up for the lowered attack rate if you took less Dexterity (though it's been a while since I used Soul Annihilation with Whispers of the Endless Paths and I vaguely recall the Soul Annihilation damage not quite being distributed equally to all enemies in WotEP's attack cone or some other quirky irregularity). And if I have a more consistent form of counterattack with WotEP, I might not need Riposte at all any more which frees me up to choose another class instead of Trickster for my source of high Deflection, such as Priest of Wael or Wizard who both also get access to Mirrored Image. That opens up some more cool multiclass possibilities, but it's getting way off topic so I apologize for the digression.