Prospects don't have to be traded, nor does anyone when competing for quality free agents. Here are just a few expenditures the Sox chose this winter over signing a guy like Rodon (feel free to substitute another TOTR free agent starter from the past four winters)
Kluber, Turner, Duvall instead of Rodon. Sox could've platooned youngsters at DH and used it to rest regulars -- like they said was planned -- and picked up an inexpensive glove-first actual CF.
Or... Kluber and Jansen instead of Rodon. Sox could've made Houck or Whitlock the official closer. They still might by the end of the year if Jansen pulls a Kimbrel.
Or... I'll be radical here: Kluber, Arroyo, Verdugo, McGuire, Pivetta, Braiser instead of Rodon. The last five signed for a combined $17M. None are sure things -- except Pivetta -- and Verdugo is a board favorite to get traded. Nobody cares about Braiser, while neither Arroyo nor McGuire has ever played a full season.