Rogues are useless in this game. When you can pick locks as a Warlock or Ranger, why would anyone play a rogue?
I was just discussing this with someone on YouTube today. Its not that I think non-rogues should *never* be able to do these things, I just think the difficulty should be so high that you'd simply not risk trusting the task to anyone *but* a rogue.....especially if your dexterity is below 17.
Making skills like lockpicking or pickpocketing uniquely tied to the rogue would inevitably lead to the Rogue being REQUIRED in any sort of optimized party composition. You would effectively be forced to either play a Rogue or always pick Astarion in your party. Making it functionally unique, as in only reliable for a Rogue as you suggest, would amount to the same thing - and incentivize save-scumming which makes for really shitty gameplay.
Rogues will be able to excel at these things through the implementation of Expertise (double proficiency bonus) and Reliable Talent (any roll 9 or less is counted as a 10). Rogues are not at all useless and don't need to hog these skills for flavor, beyond EA-levels the class differences will be accentuated evermore. In my mind; Clerics/healers have even more legitimate aggrievance as healing and resurrecting is universalised through the ample supply of related magic items.
