I think worrying too much about "future sustained success" is exactly the wrong approach. I think that's the approach most likely to keep you rebuilding.
Get the best players you can. If, for example, you are paying Melky Cabrera $10M and a rookie forces him off his position, now you're paying $10.5M for a player better than Cabrera, which doesn't kill you. If the rookie doesn't force him off his position, you still have Cabrera. If you do nothing, and wait for the rookie, you stand a good chance of ending up with neither when the rookie flames out, or gets hurt, or never makes the big leagues. Not to mention, you might be able to unload at least some of Cabrera's salary if at some point you no longer need him. If not, it's only money...no team ever loses because they spent money. Or at the least, it causes way fewer problems than not spending. There is no salary cap in MLB.
Continually shopping at the dollar store for cheap one year FAs isn't the answer...we should all know that by now.
I disagree, long term sustained success should be the primary motivating factor for decisions made by the Twins in the upcoming offseason. There is no indication that this is going to be a short window, and in that situation errors of commission are much more costly than errors of ommission. He is probably the *best* LF available, but that is a testament to the poor quality of the fas, not how good Cabrera is.
Part of it is that I just don't trust Cabrera and don't think he is worth the contract he is going to get. He is injury prone, steroid tainted, has had two good seasons, one great season (steroid enhanced), doesn't hit for a lot of power, doesn't walk a bunch (though also doesn't k too much either). I don't buy that skillset is worthy to invest in long term.