What motivation do ransomware attackers have for returning access to systems once their victims have...

 Ransomware attackers have a motivation to return access to systems once their victims have paid because they want to maintain their reputation and credibility. If they fail to restore access to systems, they risk losing the trust of future victims who may not believe that paying the ransom will result in getting their data back. This would reduce the effectiveness and profitability of their scheme. Therefore, ransomware attackers have an incentive to honor their promises and decrypt the data after receiving the ransom. References:

What is the motivation behind ransomware? | Foresite

As Ransomware Attackers’ Motives Change, So Should Your Defense - Forbes