It's important to note that we didn't need to "give another potential top player away" to get Park. The player swap came about as two separate transfers that just happened to involve the same two teams. My understanding (but someone correct me if I'm wrong) is that Clinton wanted to leave United, for the kinds of reasons you are concerned about: I think she wanted to join a squad that was more likely to win more hardware, sooner rather than later. Park was unhappy with her playing time and her role at City, but became expendable to City after they sealed a deal to get Clinton, and she came here to become an instrumental part of an exciting team on the rise. (Park definitely got her wish, and Clinton might too.) Going forward, I'm suspect you're right that some top players will be looking to move to teams that are "already there," but I also suspect there are plenty of players who would jump at the opportunity to go where they could be instrumental in helping push a team like ours over the threshold.
I also agree that we should be looking not only at veterans who can contribute immediately in the short term, but also younger long-term prospects. We did that in the last transfer window, with Rolfo at one end of the age/experience continuum, Park at the other, and Zigiotti in-between; the two most likely prospects for the next transfer window, according to the rumor mill, also include a veteran and a youngster. I think we're doing this right.