Donaldson's big issue was his health. Pillar always has been healthy. Also Donaldson actually had trade value before he was hurt, pillar has basically no trade value even when healthy.
Its not an unreasonable idea to hang onto pillar and hope hes hitting like 320/330/470 in late June and we can get something better for him. If he still sucks we lose next to nothing compared to trading him now (maybe we lose Pompey in this situation? Nbd)
With Donaldson he was actually worth something in the off season. Then we waited, hoping to get lucky and he would go on an mvp type tear but instead he got hurt and we ended up getting way less than if we traded him earlier.
Pretty different situations