Agreed. I can only imagine that the writers believe they've built Negan and the Saviors up so much that it would be implausible that Alexandria, the Hilltop and the Kingdom alone could beat them.
I still enjoy the show, but the writers are taking too many liberties that lead to plot holes and confusion. For instance, when the Saviors first show up to collect all of Alexandria's weapons: since the writers had Rick and his crew knock off that initial Savior outpost, they had so many weapons there that they could have easily stood up on the wall and taken out Negan and everyone with him. Regarding the trash people, they watched the person on the boat for how long and never grabbed him or his supplies, but one of them was willing to scale Alexandria's wall, risk encountering a whole town and kidnap Gabriel to get them? And I'm not sure why they were ready to jump into the fray after Rick's killing of Winslow. Rick was going to die until Michonne stepped in and told him to dump garbage on the thing, so it's not like he was exuding leadership at that particular moment.