I get you are a big Buchholz guy, but he is most likely gone in a year and adding more prospects simple to add more prospects isn't really a pressing need (imo) if it is ultimately taking away from the MLB roster.
Besides, there is always a decent chance you just end up having to turn around and make the reverse trade a year or 2 from now. With no guarantees that it plays out to be a push in our favor.
There are 7 starters. I don't think trading ERod or Pomeranz as opposed to Buchholz hurts the MLB team in any significant (marginal) fashion for 2017 because you're now rearranging rotation depth between relatively "equal" projections (assuming all three guys are pegged for like a 4.00-4.50 ERA) next year. I might agree with you if we had 5, but you're now stuffing two of "the backend four" into the bullpen if the season started today.
No one would deny that the two former players could draw much (much) more in return than Buchholz. It really depends on whether cutting $$$ is more important than replenishing the farm system after it's been semi-gutted the last couple years. Totally ignoring Buchholz, it also may be a misallocation of resources to burn the younger, cheaper players in the bullpen for the next few years (assuming you know 60% of the rotation is totally locked for 3+ years).
Also, it's worth noting that the Red Sox can't (shouldn't) totally gut the lower levels of the farm system because you are going to need to find some cheap, cost-controlled talent in 3-4 years when X, Betts, JBJ, etc. are all coming due for extensions. Moving someone like Pomeranz for blue-chip, AA-lower talent could help with that goal without harming the team's chances significantly next year (assuming you are like me and think he's an OK, cheap, back-of-the-rotation guy and not an ace).