Boston will still be a last place team or close to it in 2015, and possibly 2016. Boston needs a lot more than Betts and those young pitchers. They need a 1B, a DH to replace David Ortiz, a 3B that can actually hit, and development from Castillo and Bogaerts. The bigger concerns are Pedroia's decline and Ortiz's age(close to 40).
And because of several players being rushed (Bradley Jr., Ranaudo, Webster), several of the prospects values have dropped substantially.
Most free agents will prefer to go after a title now than wait until 2016-17 to win, which is how long it'll take for Boston to contend.
And you do realize our pitching prospects have significantly more upside than Boston's, right? Outside of Owens, most of their young arms have shown that they're not close to MLB ready, except for Rubby De La Rosa, and/or are likely bullpen arms (Webster).
BTW, Mookie Betts is no longer a prospect.. he's used up his prospect eligibility. And Boston isn't going to raise payroll much further, especially considering ownership has a football team to invest in, and the fact that the Red Sox are at least 2-3 years from being a threat again.