Lex's purpose in giving out quests to River and Pearl was twofold - and no, it wasn't because he was getting sentimental and missing his system. That was totally absurd.
The first reason was because over this period of time he'd basically figured out their personalities to a certain extent. They were both ambitionless homebodies who wanted to cultivate, live a comfortable life and overcome their difficulties. Such personalities weren't too bad - at least they wouldn't develop strong yet random tendencies or go off on revenge arcs.
Heck, even with strong motivations for revenge, they hadn't thought about it. If Lex were in Pearl's shoes, he would have already been planning how to make Baron Nester regret ever being born, and then discover who was the mastermind behind him.
