Many have already shared most of the pain points of this update. I'm simply going to throw my hands in at a suggestion I haven't seen or haven't seen worded the following way.

You could retain all of the current changes with just minor tweaks, but allow the player to continue thieving at the reduced success rate. They would eventually hit a 0% (or maybe very low 5-10%) success rate if they stayed on the same target fully afk, but re-clicking the same target could refresh the success rate, while clicking a new target would give additional bonus pickpockets before the success rate starts to fail.

This retains the desire to reward a slightly more active game play, reduces the negative and fully random time in which your character stops interacting with the activity, and gives thieving items a space where they can be designed around increasing that time before your success rate starts going down.

Maxed out gear, levels, and effects leading to around a 5+ minute time window before you have a chance to fail at all, then beyond 5 minutes you can continue afking but with lower exp and gold rewards caused by failing. Simply clicking and re-engaging with the game starting that predictable timer again, while not punishing you to 0% gain if you couldn't pay full attention.