I think you are overrating the PLD return. Iafallo and Kupari don't add up to much and Vilardi is still brittle. The return was good, especially considering PLD in LA last year and his contract. But it was not stellar.
Necas' value is only in that realm if he is signed long term and he is a C, which appears unlikely.
With respect to the PLD return, don't discount the effect of being rid of a malcontent. That alone makes the trade worthwhile. Additionally it should be considered that Chevy got that return with his back against the wall (again, though not of his own doing this time).
Kupari I'd agree didn't add up to much, though he's not appreciably different from any of the 4th liners we've grabbed off waivers over the years. He'll amount to something or won't.
Iafallo I thought was quite a good utility player for the Jets over the length of the season, waning towards the tail end though. I'd contend he's better than a lot of the 4th liners we've run in the history of Jets 2.0. He also adequately moved up the lineup at times.
Vilardi agreed still brittle, but time may resolve that. I'll take his performance alone over PLD for this past season somewhat negating the relative importance of Kupari and Iafallo as adds in the trade anyway.
We also always forget the draft pick that came in the draft - Alfons Freij who seemed to impress a little more than expected at rookie camp for a #37 pick. Add in the delicious aspect that it was Montreal's pick after hearing all the "we gonna get PLD for free" for over a year and it adds some nice spice to the trade. Kid might turn out to be a very sneaky pickup though he clearly won't likely be in the league for 3-4 years.
Finally cap space. Sure, PLD may yet turn into the dynamic power center that earns his salary, but that remains to be seen. We're left with a hole at 2C (or 1C depending on your perspective) but we're also not paying Unhappy Dubois $8 million.