Kinda surprised how many Leaf fans are against this. A young #1 RHD who is a few years older than your fwd core and signed to a good contract, I would have traded Hall for Pietrangelo ten times over and Nylander hasn't had any top 10 in NHL scoring seasons just yet.
I think the hesitation comes from not wanting to move another young player before we know what we have in him.
Matthews, Marner, Nylander, Zaitsev, etc. All first year players that could be huge parts of the lineup going forward.
It's hard to want to pull the trigger and move a guy before you even get a chance to see how all of these young players pan out. We've made countless errors trading away young players in recent history. Heck, the Blues have one of them (Steen).
The Leafs are finally turning around and injecting actual talented youth into the lineup. While defense is an obvious weak spot right now, who knows where it'll be several years from now when we still have these young players as cost controlled assets.
It's moot anyways, as the Blues would never entertain it, but I see the logic in not wanting to jump all over a trade like this from the other side too.