Chelsea will be in loss I guess, there's no team want Jackson except Chelsea release him cheaper than his market value. It's really rare to see old players in Chelsea, Jackson will turn 31 in the next 9 years, 31 years old striker in Premier League mostly can't compete with the young players anymore.
Maybe they are seeing that he can contribute due to his experience, in a team you also need veteran players because only young players would be a total disaster, if not you have to ask Luis Enrique when he took Spain to Qatar, they lost games that nobody imagined, however for me veteran players even if they are not so good can give the best advice to the youngsters and in part it gives balance, that's what I can think of why they want to keep him there.