What you you think about the statement that Sam is the only one who could talk Sam out of climbing into the Malack box in season 14?
It's not necessarily as clear cut as it immediately appears. 14.11 is where this comes from:
SAM: So… you came out here to see Donna, to see Mom on some – some what? Some sick, secret farewell tour? You were gonna leave, and you weren’t even gonna tell me. Me. Do you realise how messed up that is, how unfair that is?
DEAN: I didn’t have a choice! Sam, you’re the last person I could tell, the last person I could be around, ‘cause you’re the only one that could’ve talked me out of it. And I won’t be talked out of it. I won’t. I’m doing this. Now, you can either let me do it alone… or you could help me. But I’m doing this.
Who else did Dean's farewell tour not include though? Cas and Jack.
DEAN: Okay, alright. Just, you know, after what you said last night, I-I-I don’t need you and Mom coming up with some way to stop me.
SAM: You know, Mom hates this. I hate this.
DEAN: I know.
SAM: And Cas and Jack, you haven’t even told them.
DEAN: Okay, well, yeah, that’s because I’m not good with the whole big goodbyes, alright? I-I-I don’t need to get shaky on this thing.
SAM: Wouldn’t be the worst thing.
Dean went to see Mary and Donna, but he had no intention of sharing his plans with Sam, Cas, or Jack because they could all destabilize his resolve.
Sam's already ratted Dean out to Cas by the time they have the conversation above (even though Dean asked him not to tell) but Dean doesn't know that yet when Sam tries to guilt Dean into telling them (clearly seeing Cas and Jack as useful people to point to in wearing Dean down).
Dean withstands multiple attempts from Sam to talk him out of his plan. By the opening of 14.12, it's implied Sam has tried to wear Dean down multiple times already and it's not working, and he's losing hope, which he talks to Cas about on the phone.
CAS: Sam. Maybe if I spoke with Dean…
SAM: It wouldn’t matter. Believe me, I-I I’ve never seen him like this. He won’t listen to me. H-He just – No. If we don’t find some way… Dean’s gone.
Cas finally gets on the phone with Dean and immediately tries to talk to Dean about his plan (despite Sam saying it won't matter and they just have to find a way to expel Michael) and Dean refuses to have the conversation with Cas about his plan. He can't.
CAS: You’re working a case? That is so good to hear. So, I assume that means you’re not going through with it, because I have to say, Dean, this plan of yours, it was born of – of desperation, not reason.
DEAN: My plan?
[DEAN looks down at SAM.]
CAS: I-I know that I’m not suppose to know what I know, but…
DEAN: Look, I’m fine with my plan, okay? We can talk about my plan later.
CAS: Dean. You are making a terrible mistake.
DEAN: Does the name “Tony Alvarez” mean anything to you?
CAS: Y-Yes.
DEAN: Say more.
CAS: Antonio Alvarez is – is next in line to be prophet when Donatello dies.
DEAN: Okay. Thanks Cas.
CAS: Wait, Dean, Dean. We need to have a conversation.
DEAN: Look, I really got to handle this right now, okay? So, thank you, and uh… it’s good to hear your voice.
CAS: Dean?
[DEAN ends the call. He looks at SAM.]
DEAN: Really?
SAM: Dean, it’s Cas. I had to tell him.
Cas tries again when they see each other in person, and again, Dean tries to avoid the conversation, but Cas won't let him. He forces the conversation, and makes Dean stare into his eyes while saying, "So this is goodbye?" Dean can't seem to bring himself to respond, but doesn't have to, because they get interrupted by Sam.
CAS: Hey, Dean. What happened to him – that was my fault. It was necessary. But it doesn’t mean that I don’t regret it. It doesn’t mean that I don’t wish that there could’ve been another way.
DEAN: I know the feeling.
CAS: Oh, no. No, please don’t compare this with your suicidal plan. Just stop.
DEAN: Okay, alright. Why don’t we talk about that later?
CAS: Because, according to your plan, there won’t be a later.
DEAN: Cas… if you are a friend of mine, then you will understand that I have to do this and you won’t try to stop me. You think this is easy on me? It has to be done.
CAS: So, then, this is goodbye?
[SAM walks out of Donatello’s room.]
SAM: Guys. Check this out.
Cas doesn't actually get another crack at Dean before Sam takes his final shot and finally gets Dean to back down. Sam didn't manage to convince Dean in one go. He had MULTIPLE on and off screen conversations with Dean over 14.11-14.12 trying to wear him down. I also don't think Cas was done trying to convince Dean anymore than Sam ultimately was. Like Sam (in fact, in combination with him), he hoped to wear Dean down over time, which was why he was sticking around still at the end of the episode. He was not done trying to talk Dean out of this.
Also, who says what Cas got to say wasn't an effective contributor? Dean had two people who loved him telling him that they didn't want him to die and they could find another way, and when he was with them both at the end of the episode, Dean said "I believe in all of us", after both Sam and Cas insisted there was another way and made it clear that they didn't want to lose him. I don't think Cas's attempts were insignificant any more than I think Sam's initial unsuccessful attempts were insignificant. They broke Dean down slowly—though I don't doubt that Sam becoming very emotional and bursting into tears was like... the nail in the coffin.
More broadly: Sam tried to talk Dean out of a suicide play in season 3 for 10 consecutive episodes, and it didn't work. Dean had to come to the conclusion on his own in 3.10 that he didn't want to die. That also doesn't mean Sam (and Bobby) didn't help in bringing some feelings about Dean deserving to live to the surface and what was causing him to not care! But just Sam and Bobby getting emotional (even crying and yelling!) wasn't enough. Sam's best luck making an appeal to keep Dean from sacrificing himself was in 10.23 (an emotional, tearful appeal) and in 5.18 (NOT a tearful appeal—the tearful appeal failed). In early season 13, Dean was suicidal again, and actually did make a suicide attempt in 13.05. Dean credited Cas coming back as the "big win" that got him feeling some happiness and hope again. What I'm saying is Dean loves his family and I think in different contexts they all can have the capacity to make a positive difference or... a negative one in some cases. And the same strategies don’t always work. So is there a reality where maybe Cas talks Dean out of it in 14.12? Maybe, or maybe not. Maybe if Cas was the one Dean was talking to at the end of 14.11 he would have gotten the same speech. Maybe he wouldn’t have.
