The Mets don't and shouldn't give a hoot about making a division team better this season... the Mets trade meant the Mets are giving up on the season, and Robertson's a FA next year, so really has no effect except this year, and again, the Mets have given up. If the Mets want a 39 year old closer heading on 40, they can pick Robertson up next year, and it probably won't cost them too much... I don't think anyone will sign him for more than two years. And you got and deserved a nice prospect it seems for Scherzer + Cash. Acuna has great minor league numbers for a very young shortstop... plus he has a lot of speed. Heck according to mlb.com he's the 44th best prospect in all of the minors.
And markets are different for different positions. Lance Lynn is a starter... maybe several teams were looking for starters and few were offering. As usual there were and still are lots of relievers being offered... big supply.
And yes, Houston gave up a solid 25 year old prospect for Graverman. But he is worth a bit more because he is not just a rental, and he is not 39, and Houston gets to keep him for another year at a decent price, and Houston obviously knows Graverman real well since he pitched for them last year. And if Houston liked Robertson more.. hey could have had him... Houston probably wasn't willing to giv up that Catcher for a 39 year old rental.
39 year old rental.. not a horrible return imo.