Quoting: littlejerryseinfeld
You shouldn't ignore the future, but sacrificing the next 5 years because what might happen 7 years from now is a horrible way to run a sports team.
That thinking would have keep Toronto from signing Tavares at a much more demanding position.
It's always smart to think about the future, but realistically, what holes are the Leafs going to need to fill in the next 5-ish years? They have superstars; they'll find role players to fit under cap, their goalie is top-5 in the league and should continue to be. The hole is on defense, getting another top pair guy. Waiting for Liljegren might sound like the best plan, but what if he just never works out? Would they draft a guy with a 2019 1st in the mid-20s that can be an impact NHLer faster than Liljegren or Durzi? I don't think so.
The way I see it, moving some young pieces now would suck, but waiting until the team has no Marleau, no Kadri (or expensive Kadri), aging Tavares, expensive Andersen, and ELC/league min. depth before they finally round out the defense could end up being a failure.
And before the response of "we rank as a good defensive team", take a look at the shot quality that Toronto gives up. Over the long run, it will be an issue, even if it hasn't been thus far in this season. Using Micah's awesome charts (
https://www.hockeyviz.com/team/TOR/1819) the Leafs are giving up 7% more threatening chances than the league average. Tampa gives up 4% less than league avg. Boston 7% less.