First off, this proposal was offered by a Habs fan, NOT a Leafs fan!
Shea Weber is a legit #1 D. Big, strong and a bomb of a shot and also a great leader. He fits the bill for exactly the D man Toronto would like to have. (Any team in the NHL for that fact)
A few issues...he is getting older now and not sure how long before he starts to regress. Also the fact that his cap his is $12M. There is no way Toronto can fit him under that cap, knowing they have to sign Matthews (who will likely get $12M), then Nylander, Marner and Gardiner will all need contracts. From a business point in the NHL, a Weber trade to Toronto will never work.
Matthews, Marner and Nylander are all untouchable as well. Toronto can not afford to trade 20 year old stars for a D man who will be 33 y/o soon. Huge fan of Weber, but a trade does not make sense. Toronto is at the start of hopefully building a dynasty, can not trade the young bucks.