Sandin, Johnsson, and Bracco are not "scraps". Are you a hockey fan? Do you watch the Calder Cup playoffs at all?
That AHL argument literally only somewhat works for Johnsson, and until he proves his potential on the NHL stage it means nothing - so many players tear up the AHL only to fail to make the jump to the next stage. And he's 23 years old? Just cracking the Leafs lineup for the first time? Please.
Manson's skillset remains unique in the Ducks organization, and is one of the more coveted defencemen in the league. Sandin isn't remotely similar to Manson, and you clearly don't have someone in your pipeline that could be like Manson, otherwise you wouldn't be trading for him. Even if we were to "blow it up", Manson is one of the last pieces we would trade, and certainly not for prospects that MIGHT pan out.
Sure, I might eat my words. Johnsson could be the next Swedish sensation. But we have a guy who's definitively one of the best shutdown defencemen in the league. We'll take our chances with keeping Manson - we can restock our prospect cupboard without trading him. Presumably, using more Toronto picks.
For all your talking of blowing it up, by the way, Manson is 26, and Ritchie is 22. If we're rebuilding, those ages are perfectly fine.