You look for an advantageous deal, prospects aren't guaranteed, Franco was a top prospect, for example, so was Joseph.
So the better the player you have (or the higher his upside) the lower the marginal value of an upgrade (the value is the net improvement, not the gross value of the new player).
So upgrading our LF is easy, we had replacement level or worse out there, same with RF, a 2-3 WAR player at each spot is a sizeable improvement. WIth Galvis at SS, and Crawford behind him, you'll need a 4-5 WAR player to gain a significant net improvement. At 3B Franco wasn't that good last year, but has 4-5 WAR upside, so if you trade for a prospect 3B, you're basically trading one promising but unproven player for another.
Same with pitching, the bullpen is the easiest to upgrade, because after our first three we had zero WAR or worse, you're not just improving on the guy you push down, you're improving the other slot as well.
This applies to starters, but right now, if Nola and Eflin are healthy, we could have five guys who are 2-4 WAR starting pitchers, so pushing one down doesn't provide a great benefit. Adding a veteran starting pitcher, unless he's a proven #1 (and that would be prohibitively expensive) is more an insurance policy against injury (though with 7 potential starters plus PIvetta, Appel and Viza at Lehigh, how much is it worth?).
It comes down to getting the most "bang for your buck," whether it's money or prospects you're spending, and corner OF and RP are our best bets this offseason - we can obtain net upgrades at a relatively low cost with a big benefit.
