Sagan is the team’s main source of publicity, the main face for sponsorship purposes, their meal ticket. There were very strong reasons to make him the focus of the Tour team last year and this year, regardless of the fact that Ackermann and Bennett are faster big bunch sprinters. Sagan winning a stage and Green last year brought more attention than three Bennett stage wins would have. Even if Sagan wins nothing at all this year - and that’s very far from certain - him as central protagonist in a big Green jersey battle brings more attention than three Ackermann stage wins.
Sagan at the Tour usually equals success, but it always equals publicity. If his drought continues and his star starts to fade, maybe the calculations will have changed by the 2021 Tour. Maybe not. The team exist to sell kitchen fume extractors, bathroom fittings and bikes, not to absolutely maximise their sporting return.
As for Sagan chasing Green because he can’t win bunch sprints anymore, he has never been a reliable winner of big bunch sprints against the very top specialists. He doesn’t have the top speed of a Ewan/Bennett/Ackermann, but he didn’t have the top speed of a Kittel/Cavendish/Greipel either. He has always had to win against the elite through guile, positioning or by being better on a trickier parcours.