Not sure why everyone is freaking out saying Bill is leaving (or, incredibly, being fired)... it is likely much more simply that Kraft gave him a nice raise to stay in NE and Josh decided it’s better for his family. Beyond that, Josh is still quite young and he can easily afford to wait a few years for Bill to retire, especially if he’s being paid well.
Additionally, if Caserio intends to stay long-term, it makes sense for Josh to see how it’s sensible to remain here and work with him after Bill retires. Again, assuming he’s being paid well to do so.