If the Leafs did in fact have at least six guys in he organization capable of being better NHL defenders in Jake Muzzin right now, and some other team was still willing to cough up a first rounder for him? Then sure, in that hypothetical situation it'd be a great idea to trade him.
But you are the only human being on planet Earth who thinks Muzzin is this team's 7th or 8th best defenseman.
Muzzin aside, I agree with continuing to acquire picks when you can to keep the pipeline flowing. But you don't need to sell off pending UFA's at the deadline to do that anymore. The way to do it now is in the off-season, by trading surplus players who don't fit under our cap anymore.
For example, this summer one of the $3M crew of forwards (Kapanen, Johnsson, Kerfoot) will likely have to go. We could use the cap space and the emergence of Mikheyev & Engvall gives us a surplus of top-9 forwards. So there's an opportunity to recoup some draft capital after we've already had the benefit of icing our best and deepest possible lineup in the playoffs this year.