Couple things.
1) You have Morgan Rielly as a UFA. He was a draft.
2)You have cases like PK and Weber who were traded for each other. In other words, you have to draft a top pairing Dman to trade for a top pairing Dman. So that really is "draft one", not "trade for one".
3) Look at the quality of the top pairing dmen by how they were acquired. Most of the top of the top were drafted: Doughty, Pieterangelo, Karlsson,Keith, Letang, Hedman... etc. Now look at the UFA guys, MOST are "meh"- Mike Green, Niskanen,Krug.
4) Some of the guys who were traded for or signed as undrafted UFAs were "lucked into": McDonagh, Muzzin... so you can't really say "We don't need to draft a #1 D, we can just sign an overager out of the CHL and he'll turn into a top pairing guy". It happens, but you can't plan for that.
You can surely add a top pairing guy through trade or through UFA. People who say you can't are wrong. The problem is the price you have to pay for guys that way.
