You make a good point about attainability. We all know that Asik seems very available, but what is Sanders' status in that department? He is signed to a pretty ridiculous contract after a worst case year scenario. With Henson making a name for himself, wouldn't it be reasonable to assume that Sanders can be had for someone just taking on his contract (especially with the number of non-guaranteeds we have).
It sounds like Sanders' trade value has bottomed out. So Milwaukee have taken the point of view that is the wrong time to trade him. They can't get anything close to comparable or even adequate return for such a talented young big man. So they are going to hold onto him for now and try to rebuild his value (and likely offload him further down the road) unless some team ups their offer considerably and makes it worthwhile for them to part ways with Sanders.
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Edit: My best guess for what type of trades they would deem acceptable:
(1) A low lottery pick (#10-14) + cap relief
(2) A good young prospect (H.Barnes type) + cap relief
(3) An established pro, a quality starter, who is young enough to be part of a rebuilding program (J.Green type, C.Parsons)
I would think Boston's best offers would be a (a) straight swap of J.Green for L.Sanders (b) #17 + next year's lotto protected pick and cap relief for L.Sanders + 2nd rounder (c) Olynyk + #17 + cap relief for L.Sanders + OJ Mayo.
I am not sure they'd be interested in Olynyk with Ilyasova there though.