I agree with a lot of what you just said there, but I think Dickey has only a few years of top notch performance (I'm not doubting he can pitch for at least 5), so to me getting a great deal in return for him is worth it. I completely understand what you are saying about Dickey having a strong connectivity factor with the fan base and with you personally, but, strictly speaking for myself, his captivating qualities and appeal are what will make me be a fan of his forever, no matter what team he is on. I'm not worried about losing him in that sense.
I agree with you about Gose, and Arrencibia, but I think Arrencibia has a chance to become a little better (although still not a centerpiece for a trade).
With Dickey, I disagree with you about making them relevant. Even with him they only won 74 games so I wouldn't call that relevant. He only made them a threat on the days he pitched, we would need a couple more R.A Dickeys to be relevant on a team like this.
I don't see where this bullpen is going to be any better in the next couple years or our outfield for that matter. Given, as you also pointed out, the limited flexibility that Alderson has with payroll, how else can he make the team better long term if not through trade? The problem with Dickey is he isn't a long term piece, by the time the Mets may have their stuff together, he might be just an average pitcher.