Both those prospects are behind players that are 22 and 23 years old. Margot would be stuck behind Betts, and Guerra stuck behind Xander. At some point you have to use your prospects to fill current team needs. Sox needed to upgrade bullpen. Kimbrel is easily a top three closer in baseball. The Royals have gone to two straight WS based on their dominant 7-8-9 inning bullpen guys. This puts us closer to that. You have give up something to get something. Margot and Guerra are 2 to 3 years away from being in the show. If anyone thinks that teams are going to give us top players to fill the Sox needs without getting something back you are dreaming. Finally, you would only have Chapman for a year, while Kimbrel the Sox could have control of for the next three seasons.