They used way more relievers this year than last year. That was actually the problem in that three of them failed. And last year they had Kyle Gibson in the Lorenzen spot and Falter in the Walker spot. Only difference is they made the mistake of letting Falter pitch. There's nothing they could have really done differently. Walker's roster spot could have gone to Marte but that doesn't mean you would have wanted to see him in a game. Kerkering might have been swapped for Covey next round, but so what.
Bench should have been upgraded x 2 (someone better than Cave, someone better than Wilson or Pache) but a key PH or spot start was hardly going to make that much of a difference.
These decisions were both made at the deadline. That they had enough relief pitching (including Lorenzen, and I suspect even then they knew Kerkering might be in the mix) and that they were willing to ride with Rojas (to the point where they passed up several RH OFs on waivers in a month later too).
Walker's a weird one. If he was injured he should have and could have been replaced. At the end of the day the reality is the team's biggest free agent pitching signing wasn't good enough to start and wasn't comfortable relieving. Would the Sanchez start had gone differently had Walker made it? Probably not (though we might not have had to see a blown save).