Ellsbury was the Sox #1 prospect at a time... and then he actually turned into what prospects are supposed to turn into-- a good player. Prospects might work out, but they might not. Smoak's value went way down before he got to Seattle. He's hitting .159 in Seattle-- top prospect potential I know. That's why its always better to go with a sure thing. Or, Ellsbury can go to another team for more prospects, whatever.
And leverage really doesn't have much to do with it. Teams know when the ship is going down, and they must know their pitching staff won't all have career years again. Plus, the Giants/Dodgers/Rockies are not going to make it easy for them. Plus, most of the big market teams already have a great first baseman, so there aren't as many buyers as one might expect. If they think they'd rather have a first round draft pick than a package, then they can take it.
And Lars has only been to AAA once. His July was a great sign, and I think its just a matter of time. But if Hoyer wants Rizzo instead, they can have him. Two years of A-gon gave the Padres a window, but eventually they'll need to get what they can.