Nothing is ever guaranteed on absolutely anything in life, except for the fact that we're all going to die. But in baseball (and this is a baseball related discussion), barring physical decline due to age (inevitable), skill erosion, or life-altering injury, the best way to predict a player's future performance is past performance. There are exceptions, like a pitcher altering his repertoire, or a player altering his swing, but there are decades of both qualitative and quantitative data to support this fact. Nothing is guaranteed, but you need to have a baseline to evaluate MLB talent, and that is it.
Agreed with the point that they are doing the lord's work with establishing their next core, but their inability (hopefully past) to identify worthwhile veteran additions is what's worrisome, and the crux of my argument.