I get this, but right now the Leafs have three top-4 defencemen and 4 bottom-pairing defencemen. Which, to me at least, means their blue line is not Stanley Cup-winning caliber. If Sandin can work the mental mistakes (mostly brutal turnovers and ill-advised rushes) out of his game, Holl doesn't fade late in the season, I'll be more optimistic.
Even if you think they're a bit better than my take, if Holl was exposed and selected by Seattle, the Leafs (injury-free) blue line for now is:
Reilly-Brodie
Muzzin-Dermott
Sandin-Liljegren
Two years from now, if Sandin and Liljegren keep progressing, Dermott likely is moved and another veteran is thrown in there, I see a solid, consistent blue line that can help carry this team deep. But this season - another critical one for this team - as unimpressive as Holl has been so far I think if they lost him Keefe would have very little wiggle room, esp. if, as almost a certainty, any of those six misses significant time due to injury.