This is one of those trades that just doesn't ever happen. A deal around Subban + Turris should net you at the very least an under 25 bonafide #1 center in terms of value. However, in the salary cap era no team is perfect, or even all that close, so even if the value is fine, big trades like this don't happen because no team can really spare the pieces required to make this sort of deal.
Say Toronto decides "hey we can do this deal and fix our D". So let's just for fun say they do Matthews for Subban + Turris. Well now their 3 C's are Tavares, Turris and Kadri. Still strong, but now there are a lot more teams that match up well down the middle against them and it's not longer as big a strength they can exploit. Adding Subban to the D will drastically improve them, but it doesn't vault them to the top and teams like San Jose, Anaheim, Probably still Nashville, Tampa Bay, the Hurricanes, all still have a stronger D core, and other teams aren't far off. So now they are in a position where they. So now you have a decently balanced team but still not perfect, and now you need a coaching strategy that is adaptable so that you try to exploit your strength on D against teams like Pittsburgh, Washington, and Florida, but then try a strategy that exploits your depth at C against teams like Boston, or Tampa, and even in those cases your advantage is marginal. Personally I would rather run on a strategy where you just try and exploit your advantage at C against the vast majority of the league, and the couple of teams who you don't have a big edge there, you will likely still gain the upper hand by having more talented wingers than Pittsburgh. It doesn't guarantee a win, but if you can figure out how to consistently exploit your advantage, you will have an easier time that constantly switching your strategy.
For Nashville they are in the same boat but reversed. Matthews, Johannsen, and Bonino would be no where near the deepest C pool in the league, and now Nashville would not have the deepest group of D either. The team would be more balanced, but they would have to continue to tweak strategies depending on how they matched up against opponents, as there are a good number of good teams that either have better Centers (overall), or better D. Again, it makes the strategy more difficult to be consistent since the edge you have changes depending on who you face.
To me this is one of those NHL 19 trades. The value isn't the problem, it's just that it's a massive shake up that doesn't necessarily make you any better overall since the give and take is equal. In a league with a salary cap, it's nearly impossible to create a team that has elite players at every position and is deep, so teams try and create a competitive advantage somewhere, and then work to create a strategy that allows them to exploit their advantage more than their opponent can exploit their disadvantage. A team that doesn't' have that will have a very hard time competing against the other top teams in the league.