As I noted, rosters expand in two weeks so it's not really Papelbon displacing anybody for any significant time. Indeed, Cubs now at 7-man bullpen when club has been at 8-man bullpen for bulk of the season. There is room.
So, issue is: can he help? That's the issue.
Comparing Papelbon to career Papelbon is irrelevant. Issue isn't about bringing in long-time successful closer but maybe bringing in a guy who can be useful on a 7-man or 8-man bullpen---and paying major league minimum without any multi-year commitment.
As to pitching in Wrigley, it is 93 on a scale of 100 as average pitchers park. It's been a decided pitchers park this season. Also, irrelevant to the issue.
No, Maddon is not going to be too interested in Jose Rosario over Papelbon in September, as you suggest. Probably Papelbon gets better deal elsewhere but if he's here, I'm on board.