Trading for a top 4 Dman this season would force the Leafs to leave Carrick or a decent forward unprotected in the expansion draft. Also, any trade for a top 4 Dman would probably involve JVR, so there's that to think about, too. Right now they'd choose the 7F, 3D, 1G option because they only have 3 Dmen of any real value. If they traded for a 4th Dman of value, then they'd choose the 8 skaters & 1 goalie option.
No trade this season:
7F: JVR, Kadri, Brown, Bozak, Komarov, Leipsic and one of Martin, Rychel or Leivo.
3D: Gardiner, Rielly and Carrick. Only other Dman that needs protecting and is worth anything is Corrado and he's not better than Rielly, Gardiner, or Carrick.
Trade this season:
8 Skaters: Rielly, Gardiner, Mystery Dman, Kadri, Brown, Leipsic, and two of Carrick, Corrado, Bozak, Komarov, Martin, or Rychel
Carrick, Bozak & Komarov >>> Corrado, Martin, Leivo and Rychel.
So, if the Leafs were to trade for another top 4 Dman this season, they'd lose one of Carrick, Bozak, or Komarov for NOTHING. I'd rather them wait and only lose one of Martin, Leivo, Rychel or Corrado in the expansion draft.
Guys like Matthews, Nylander, Marner, Hyman, Soshnikov, Gauthier, Kapanen, and Zaitsev are exempt from the draft entirely. (Not enough pro experience)
Side note: Every team needs to leave at least 1 goalie unprotected and the Leafs currently don't have another eligible goalie signed past this season. Look for one of Sparks, Bibeau, or McElhinney to be given an extension before the expansion draft on June 21st.