I don't agree with the notion that anything is "solved" therefore you shouldn't go after a specific position. Tavares is an upgrade on at best 11 of our forwards, and at worst, 9 of them. There is no universe where adding Tavares doesn't make the team significantly better. I mean right now Zach Hyman gets top ES minutes a game. There is unquestionably enough ice time for Tavares. Its like saying Pittsburgh shouldn't go after McDavid if they had a legitimate chance to get him because they already have the best centers in the league. Who cares? They can all play.
Trying to shoehorn your plan around needs is how you become Edmonton giving up the opportunity to have better players for a guy like Larsson or Lucic. Yes we would like to have an elite RD, but none are available, except maybe Karlsson (who the Leafs should 100% go after as priority #1 if possible). So does that mean you just don't try to improve the team? Or that you take a move that provides less gain and upside just because it fills a perceived need? Does signing Calrsson for $9 million make more sense than signing Tavares for $9 million because of positional need? No. The later is a better addition.
There is no magic equation to winning, e.g. 2 C + 1 W + 2 D = Cup. Its the teams with the best players that are the best. Look at Nashville. They hardly needed to add Subban with Josi, Ekholm and Ellis, and needed help up front. But they picked up Subban anyway. Because he is an awesome player that significantly improved the team. Pittsburgh didn't need another scorer. But Kessel was available to them so they jumped on it and is significantly improved the team.
Think of WAR. If you have a replacement level middle infield, and 3 of the best pitchers in the league, adding a 5 WAR pitcher is still more valuable than a 2 WAR second baseman, even though you "need" a second baseman more.
The teams with the best players are the best teams. And in the cap era even the best teams have holes. You should try to build the team with the greatest strengths, not the most filled holes.
The reality is, with the Leafs cap situation, they likely cannot sign Tavares for 7 years $12+ million. But if they can get him for a number that does work you will be hard pressed to make a more impactful addition. it is the same reason why they went after Stamkos even though they had Matthews-Nylander-Kadri-Marner-JVR etc. And they didn't even have Zaitsev, Hainsey, Dermott, Liljegren, Borgman in the picture on the blueline at that point.