I'd make this trade in a heartbeat, depending on how much Knies gets paid this off-season it could honestly look even worse. Rantanen is a bonafide superstar and is actually built for the playoffs, if you knew Marner was walking regardless you'd think that would incentivize you even more.
Look at where the leafs are now, they're losing an 100 point all situations player and are going to have to try to replace his production in one of the worst free agency pools we've seen in years.
These are not mutually exclusive lol, you need good depth but you also need your horses to be your horses. Carolina is like the quintessential example of this - a team that's lauded for both it's forward and defensive depth that hasn't won an ecf game. You need gamebreakers to win.
Take the flip side though... lets say they got Rantanen, and did a sign & trade for $13m x 8....
For arguments sake, let's reasonably predict that they wouldn't have Carlo, but do something like Minten + a 2nd for Laughton. They would have $15m in cap space going into this offseason.
Their right side would be Rantanen, Nylander, Jarnkrok and Robertson.
Their centre depth would be Matthews, Domi, Laughton, Kampf.
Their left wing depth would be McMann and Holmberg.
Let's give Tavares $6m of that $15m.
Yes, there's ways to squeeze a bit more money out of Kampf, Jarnkrok and Domi... but you're essentially trying to go into a shopping season with $9m to get yourself a middle 6 centre, a top 6 LW and a top 9 LW, and a top 4 right shot defenceman... with basically no draft capital to do it.
To me, that's a somewhat impossible task.
Yes, they are going to have to replace Marner's production to an extent... but we should also come to the realization that its not just those 4 that couldn't win... it's what those 4 did to the salary structure that prevented winning... so it's not as simple as replace Marner with Rantanen and off we go. We need depth.
The Leafs have theoretical pieces who can do some of what Marner did if given that opportunity. Domi has shown great flashes of brilliance on Matthews wing. They have a guy like Cowan coming in who obviously isn't Marner, but brings creativity from the right side for $900k and a ton of upside. Neither of them are going to penalty kill, but that's what your 3rd and 4th line should be for. Neither are going to end up playing a regular shift on defence because you get into roster issues for 1 game... we can live without that.
This is still a team with a ton of skill. They have Matthews who is likely the league's best natural goalscorer; who happens to also be an extremely solid two-way player. They have Nylander who is coming off 3 consecutive 40 goal seasons.
The Leafs do not have easy access to somebody that can replace the loss of Matthew Knies' skillset... and his skills are already in somewhat short supply on the Leafs.