I'll say this: To go to any team and say not only do we want you to pay a 32-year-old Josh Donaldson $20 million + for 1 year, we also want your top prospects in return is a tough bargain no matter how good he is. The Donaldson trade looks good on paper and he's clearly worth top prospects, but sometimes trades on paper don't make practical sense.
3B is a stacked position around the league as it is, and it's true that Donaldson's bat will play anywhere, but a huge part of his value is derived from playing a premium position on the field.
Realistically, which team:
A) Has the prospects to afford Josh Donaldson?
Has the salary and payroll to afford Josh Donaldson?
C) Has a clear need at 3B or the infield?
D) Is going to be a contender in 2018?
Add that to the fact that Machado might also be available in the offseason who's younger and going to be cheaper. You've got Frazier and Moustakas as free agents who can play a competent 3rd for any contender. The stars seem to be aligning against trading Donaldson because the market might not be there. And if you realistically can't move Josh Donaldson for a return that makes sense, it only makes sense to try and contend and not waste his final year.