The bank wants its on-shore employees to be gone.
Many (most?) of these same employees hate their jobs but are unlikely to leave due to the current job market. It's hard to spend all day in a job you hate and then find the mental/emotional energy after getting home, dinner, etc to job hunt. It's not impossible, but difficult.
Enter severance.
If the bank is worried about high performers leaving, they can stop worrying. Disengaged employees are not high performers. People perform highly when they like their job.
Nearly all employees that would take the voluntary buyout are exactly the employees the bank wants gone anyway.
The bank keeps hoping that people will leave on their own. It's not going to happen. Wells needs to take lead on this.
Until then, the armies of unhappy employees will continue to do the bare minimum. I don't understand how the bank sees this as the better option.