As an admittedly huge fan of Weber (though I wish we never traded for him) lets look at the facts, shall we?
Cons:
Weber is 33 this season.
Weber is coming off of a 26 game season, and a surgery.
Weber is signed for the next 6 seasons after this one.
Pros:
Weber is absolutely a 1D
Weber absolutely is our best defenseman
Weber absolutely will continue to be elite for at least another 3-4 years
Weber could very likely end up like Chara, and contribute at a top 4 pace for the rest of his career.
Realistic problems for either side:
We trade him, and he becomes Chara 2.0, we just lost a RD for the next half-decade.
We trade him, and he falls off after two or three seasons, we just nabbed a top 6 winger for the next decade for two years of Weber and 4 years of wasted cap.
We don't trade him now, and he falls off in two or three seasons, we are stuck with a degrading defenseman, or we trade him for three quarters instead of a dollar.
They should rather let Liljegren develop than trade Nylander for Weber.
Lilly hasn't played a single game in the NHL, and there is no evidence to suggest he will be a Top 4 defenseman within the next two seasons, which is the window that Toronto should chase as much as they can with ELCs. Lilly could (though I doubt it) end up being Zaitsev 2.0 and not really turn out in the NHL as expected. That would leave Toronto with a lot of wasted time, and a terrible right side --- they need to address it as soon as they can if they want to make it past the first round, and unfortunately a #1 defenseman, regardless of age, doesn't move very often. Weber is one of the few possible players the Leafs could realistically target.
The guys seems to always make bad proposals for Hab players.
The Habs free up a ton of cap space and add to a position of relative weakness.
They can offer Carlson 7m to fill Weber's void, and still come away with close to the same cap-hit while Nylander is on ELC.
Draft a winger (Tkachuk, Zadina or Svechnikov) to play 3RW, and move Shaw/Patches
Chucky, Drouin, Lehkonen on the left, Nylander, Gallagher, Draft Pick on the right --- the Habs wing depth becomes absolutely f***ing ridiculous.
Get lucky and snag a 2C + LD in a trade for Patches + our 2019 1st, and sign Tavares and that team becomes a cup contender overnight.
Don't kid yourself, Weber is great, but freeing up that cap gives the Habs an abundance of options with the caliber of players hitting the market in the next 400 days.