3yrs younger isn't that much of an age gap when it comes to need. Frost is a natural center, it's not a secret how much the Leafs need and want a C that can play in the top 9 and especially slot into the top 6. You don't take less when you are in the position of strength in a negotiation.
So Robertson is a crap proposal, I said Knies because the value of Frost to the Leafs and the player he is, would be something like that. A former 1st that was trending in the right way before being Tortorella'd isn't crap value when other GM's know the value and they can try to buy low on him because of his start, but Briere would be laughed out of Philly for trading Frost for something like Robertson, not unless a guaranteed 1st is coming with him. Also Chicago being interested, they'd probably give up a better value for Frost and out of the conference, so if the Hawks are serious, I don't see Toronto landing him at all.
It's wild that Briere is sticking to Torts right now when Montgomery is available, Torts is an epic dipshit, he's ruining players just so he can have his way with how he wants them to play and "React" to his idiotic methods to send a message or whatever that moron thinks he's doing as a coach with his dumb old school methods. I mean as a Pens fan, kind of fun to see it go down that way, but for the sport, to see prospects and players get ruined by an idiot like him is bad for the game.
And btw - I am a fan of neither team, as a Pens fan, I despise both teams equally at this stage, as well as the Hawks.
Here's the challenge with Frost...while he is a natural centre (which are generally more valuable than wingers), he's also somewhat stuck in "no man's land".
Yes, he has 2 seasons with 40+ points under his belt, but he's also got 245 games NHL experience, and is has not shown the ability to take that "next step" despite ample opportunity on a Philadelphia team that is very thin up the middle.
Let's also not forget, 40 points isn't quite what it used to be given the increase in league-wide production. Amongst centres last year, Frost ranked 74th in the league... and he's not really "gritty" enough to be a prototypical 3rd line centre.
If Frost cannot take that next step and figure out how to put up ~60 points, he's potentially not all that useful to a good team.
Knies on the other hand, recently played his 100th NHL game. He's a solid 2-way player and obviously plays a power forward's game. The list of teams that wouldn't love to have him as a 2nd or 3rd line option on their wing is probably pretty short.
Ultimately, I think you have to look at Frost's trade value in a similar context to Timothy Liljegren. 25 year old players (Liljegren 2 weeks older) that have shown a bunch of promise at the NHL level, but unable to put it together and take that next step to where they cement themselves towards the top of the lineup, and they don't really play the right "kind" of game for a bottom of the lineup position.
The challenge of course in trading a player like that, most of the interested teams are going to be ones that look at him as "lets take a shot and see what happens", and they tend to be stingy when it comes to moving draft picks / prospects because they are rebuilding.
EDIT: FWIW, this isn't me saying Robertson for Frost... I know that makes absolutely no sense. This is me saying Liljegren for Frost would have been a theoretically interesting deal.
Given that is obviously past, and that the Leafs have a very crowded roster up front and the Flyers don't actually need to downsize the roster like the Leafs did with Liljegren.. maybe something like the 3rd Toronto got, plus one of Holmberg, Kampf or Dewar.