The Yankees' quick turnaround has everything to do with the shrewd acquisitions of guys like Castro, Hicks, and Gregorius for virtually nothing and seeing them blossom in New York; not "blowing their brains out" in the IFA market which has largely produced nothing significant for them yet.
You keep bringing up the failure of not going "all in" in the 2015 IFA market without considering that the Cubs spent $15.53 million that year and could do so because they were in the penalty the previous year and thus had the whole year to reach agreements for the 2015 class. The Dodgers also spent $7.88 mill that year (this is looking at just the 16-year old kids, not Cuban signees). The point is, there are other teams signing these players as well, so its not as simple as deciding to "blow your brains out" and then everyone signs with you. I mean, we could have easily signed a bunch of $300-500K kids for $1+ mill and made it look like we were getting sweet talent, but spending a ton of money and actually signing the best players are two completely different things. If you look at that class, the top ~8 or 9 guys all signed with different teams, and while the Cubs went crazy, they didn't land ANY of the Top 3-5 elite guys. Why didn't they sign Vlad Jr. if they were going all in? Probably because he had a deal with us for maybe 1-2 years prior.