I don't get the issue given my post above. Walker is arguably a better player both offensively and defensively than Murphy (Walker has a career 113 OPS+, Murphy a 110+, and we all know Murphy is a shitty defender), the Mets like Herrera, and they got an extra first round pick out of the exchange. Herrera will be 22 when next season starts, it makes a lot more sense to block him for one year as opposed to three or four. He lost prospect status because of his MLB playing time last year, but BA had him as the #46 prospect in all of MLB going into last season.
People really need to constantly keep in mind that more than half of major FA signings end up being mistakes, maybe as many as two-thirds. I don't think the Mets are broke or crying broke, they offered Zobrist more than he ended up taking from the Cubs (4/60 to 4/56). I think it is very smart to be exceedingly careful with FA signings these days (although I have my doubts about Zobrist being a good investment at this point), and clearly once Zobrist chose the Cubs, the Mets preferred the Niese for Walker move over signing Murphy or Kendrick to 3 or 4 years.
The Cabrera signing seems pretty dumb to me, though. They should have been prioritizing defense when adding another SS, and Cabrera is the opposite of that.