Petry value at its highest now, having taken center stage over two seasons. MON can afford to make Petry extremely attractive because he has two seasons beyond this one on his current 5.5M contract and MON is in no way in a Cap crunch situation over that span of time.
If Bergevin holds back 2.75M of Petry's contract (the maximum he is allowed to hold), it allows a Cash-strapped contender three playoffs with a #3 RD that can solidify their D for a long playoff push. Just how valuable is that to a team like TOR that cannot keep Gardiner, for example (Hainsey, I won't even mention because he is 100 years old, anyhow), and must find too for Matthews, Marner and Kapanen starting next year. Apart from Kapananen, maybe, these are not bridge contract candidates and their long term contracts will likely be superior to Nylander's contract in both value and term.
Two years after this one, Sandin, for example, will already have begun to make his mark on the team and trading Petry at the deadline will be an option as TOR will likely not be able to afford his full price.
At least, TOR will have gotten three years of a tangible upgrade at D for a LHD in Gardiner (that they weren't going to be able to re-sign under their Cap anyhow as an UFA) and some draft picks.
Next year, in theory, Petry's value is still pretty good, but things get more complicated for the type of return that MON can get, especially if it is shooting for a Sandin instead of a looming UFA. At the point, the expansion draft will be on the horizon and teams will be hard bent on passing along young players that will need to be protected rather than players like Sandin that are exempt from the process.
Mind you, with Gardiner on the team, MON will likely protect Weber, Gardiner and Juulsen. Mete will be left unprotected, undoubtedly, from the players of any worth on D. Given that, in this scenario, I would trade Mete as well next season, at the latest, as part of a package for an upgrade at wing, perhaps.