My reasoning is pretty similar to yours
The risk of an 8 year deal is that Matthews leaves/isnt a capable best player to lead a team to a cup(which despite his repeated playoff failures so far, he still is good enough to win with, he needs to find a way to put it together come spring)
6 year deal is best case, as if Knies is a 55-65 pt PWF we can sell him off for a lot with 2 years left is Matthews walks to kickstart the rebuild. Teams may not want him at 8 year term (4 years left) as its a lot of term and risk of his next deal being one where PWFs fall of in year 2 or 3 of
It also works if Matthews extends, nd the team has won or is close to winning by then as 2 super cheap years at 4.5M before we can extend him to a bigger deal at only 28 years of age vs 30 (2 years could be huge if he falls of at 32/33 and we give him 7-8 years his next deal)
I don't think you get 6 years out of Knies at $4.5m.
Everyone knows the cap is going to jump immensely. Auston Matthews is the epitome of doing that with the 4 year deal so he can get another hit at it.
If he's listening to Auston Matthews (probably, given that probably idolized Matthews growing up), he'll likely want a much higher premium for giving up years 5 and 6.
Personally, I can see a world where you can get Knies at $4.5m x 4.
To get 6 years out of him, it probably needs to be a $5.5 to $6m deal... just based on what Kaiden Guhle ($5.5x6) and Guenther ($7.15x8).
If you're the Leafs, does it make sense to pay an extra $1 - $1.5m for another 2 years? I guess that comes down to whether you think you think that extra bit of cap space might be the difference between winning and losing over the next 4 years.
Personally, looking at where the Leafs are at in contract situations and ages with Matthews & Nylander, I'm a bit more inclined to go 4 years.... especially if he's subscribing to the Matthews school of thought and is adverse to doing an 8-year deal. Most of the teams doing these types of deals with young players aren't very good today.