Adding McD while subtracting Rielly, Kapanen and a 1st doesn't make the Leafs a better team, let alone a much better team. With that trade they might be slightly better next season, while the team is still pretty early in the rebuild, but the loss of Kapanen would make them a worse team by the following year (when it is likely that at least a couple of JVR, Bozak, and Komarov are gone) and then it is downhill fast afterward. Not that it matters because no GM would ever consider it for a second and Lou certainly will not.
McD played 28 minutes 3 times this year.
Their ES ATOI last year was 18:48 for MR and 18:53 for RM
Their PK ATOI last year was 2:21 for MR and 2:21 for RM
Their PP ATOI last year was 0:58 for MR and 3:05 for RM
So, essentially the exact same, except for McD playing far more the PP. The Leafs have zero need for PP help. Is McD the better D at this time? Yes. Is he a good fit for the Leafs? No. Would he be more valuable over the next two years than Rielly will be for the next 5 years? Not a chance.
Despite many Leafs fans being obsessed with acquiring a #1 D, or at least another top 4D, it seems pretty likely that the Leafs management has decided they are not going to trade significant assets for a D. The shopped around and found the price far too high (and that was situations that were far, far, far, far more reasonable than the trade suggested in the OP). That ship has sailed and is why they signed Hainsey to play on the top 4. They will develop from within. They are a high scoring team, with decent goaltending, and they can still be a playoff team as their D prospects mature over time. Of course, I could be wrong, and perhaps they will trade assets for a #1D who will stick around for a year or two, but that goes against what management has said they are going to do.