I think that there is one difference between the two sides of this argument. It comes down to this:
People arguing against the trade are looking at how the trade looks today; using the numbers we already have to work with.
People arguing in favour of the trade are looking ahead to how the team will perform going forward.
Prior to next season, it is hard to argue anything but we lost a player with more trade value. I.e, we could have got more in the trade, making this a bad trade TODAY. My point is, the present is the only concrete thing we have to work with, so leave the future where it is. Today this is a bad trade because we lost a more valuable assets with nothing to balance out our lost value. That's the against argument.
If you look back, very few from either side of the argument have stated that they think the team will perform worse next season. I certainly think the team will improve upon last year. But it's hard to know how the team would have performed had the trade not been made. Attributing team performance based on the trade screams hindsight bias. The best statistics to get an idea of the impact of an individual are ones like relative corsi, but most supporters of the trade tend to dislike them. As for the intangibles, they are just that, intangible. In my opinion both Subban and Weber bring intangibles. Probably both will benefit their teams. Subban's energy, Weber's leadership. It's hard to make solid arguments for them because of their intangibility for lack of a better word.
But still there is always the retrospective aspect of a trade too, of course, once those post-trade numbers become reality; but we aren't there yet. Perhaps Weber has the next best 6-7 years (wanna bet?) and then we'll be able to say which side of the trade played out better. That doesn't change whether you lost out on a trade in the present. This is also why I think the argument for the trade is pretty weak in the present. Hopefully for the Habs the argument becomes much stronger in the near future.