He is worth it in the sense that he was a UFA, and UFA's have an inflated price because you are bidding against other teams, and getting to add a significant asset for nothing other than cap space.
However when you compare him only with the production per dollar compared to other players, even on his own team, it won't look as good. Is he going to justify a cap hit that is over double Kadri and Rielly, or is his cap hit for the production over the life of the contract going to compare well to whatever Matthews signs for? Likely not.
With UFAs, it's almost as if part of the price paid is giving up cap space for the asset, and I would much rather spend $11M on Tavares then $12M for a combo of Bozak and JVR, or $10M on Gudbranson, Beagle and Roussel.
So I think if you are going to go and spend a chunk of your cap on acquiring players as UFAs, Tavares is a far better value than most teams will get.
I think the trick is realizing you can likely only afford to dip your toe in that market a couple times, so you might as well make them count.
I wouldn't read too much into a cold streak, without them Tavares would be a 100+ point player every year, and McDavid would be putting up 150+. They happen, and the Leafs are likely going through some growing pains because of some volatility in their line up. They are missing 2/3rds of their projected top line, Tavares is new, and a revolving door on the 4th line and bottom pair. At some point, there was likely to be some struggle. They have been an uneven team so far, but the good has outweighed the bad. If they can start playing most of the time like they did against Dallas, and the first game against Winnipeg, and less often like they did against St. Louis, they will be as advertised, a contender.
As for the salary cap repercussions, I don't think Tavares contract has really changed anything. The contracts to Marleau and Zaitsev are far more harmful. The Leafs are playing hardball with Willy because they need to get very good value out of their RFAs (as Tampa, Nashville, Winnipeg have done), and Willy is first up. If they didn't have Tavares they wouldn't NEED to do so, but they would probably still want to save space to upgrade elsewhere, so I don't think the current cap situation is impacting this negotiation as much as the fact that the Leafs feel they need to set a precedent, and Nylander doesn't want to give a big discount to the Leafs to be their best trade chip.
There isn't as much pressure from either side to crumble just yet. The Leafs have been winning with Nylander out, and every passing day pushes more of the cap hit to the 1st year, and reduces the subsequent years which is in the Leafs favour anyway. Nylander isn't losing a ton of money, he knows that the only thing stopping the Leafs from signing him is AAV, so he won't have any trouble getting them to offer year 1 as virtually all bonus once they are comfortable with the term and AAV going forward.
There will be a point in November where the Leafs could sign Nylander for over just over $7M per, and it result in a cap hit of less than $6.8M in all the future years. I am not sure how far apart they are in terms of the numbers both sides would actually be OK to settle at, but I would have to think that if it got to a point where the future cap hit came down to where the Leafs were comfortable, but the real total salary was where Willy wanted, that the contract gets done, and nobody draws a line in the sand based on principle.
If the gap really is too much to bridge, Willy is an asset, they don't lose him for nothing. Tavares at $11M plus whatever you get for Willy (provided a reasonable trade) is worth more than Willy plus cap space you can't use right away.