Or they could roll that $12mil into next year so they can afford to pay Adams what he is worth. They also have to worry about resigning Burnett and House if they want. NFL teams with sustained success generally look more than one year in the future too, so planning for the following year is probably on their mind - they’ll have Matthews, Cobb, HaHa, and Montgomery due for new contracts. And then of course in 2020 there is Aaron Rodgers’ last big(huge) contract, Mike Daniels with another big one, and if Blake Martinez keeps it up there’s another big one.
The Packers have a bright future with their current roster, but only if they manage their salary cap to be able to keep these guys. A hit of either $4.5mil or $3.7mil would likely mean letting at least one important free agent go next year. In addition, with it requiring a trade, you’ll be giving up a draft pick for half a year of a past-his-prime veteran.
In my opinion, even if it was free I don’t think adding either of those two players to the roster would greatly improve the defense. We can certainly debate on that point. But I don’t think there is any debating the fact that they are definitely not worth their weight in cap/draft pick value.