That is correct, but doesn't guarantee that it will always stay that way. Does anyone even know what kind of football shape Mack is in? What kind of player will he be after he gets a check for a guaranteed $60M?
No one knows what type of a player a guy will be after he gets paid. Personnel departments should have better ideas than most, given the level of homework that they do on these players during the pre-draft process and after they hit the league. Conveniently, Green Bay's personnel department will be informing any decisions on acquiring him. I obviously have no special knowledge of what type of shape Mack is in right now, but what I've read/heard reported on him is that he's in impeccable physical condition and that he's always been that way (i.e. he stays in top shape regardless of whether he's with the Raiders or away for the offseason). That's at least his public reputation. I see no reason why it should be in any doubt.
Perhaps some clues as to Mack's drive: Never missing a game. Surely he's been dinged up; all NFL players get hurt to some extent. That he's never missed a game means that he plays through it. I'm not trying to say that he's secretly been playing through torn ACL's or something crazy, but a player without drive won't push through discomfort. Secondly, he's been on three teams with losing records and he's turned in stellar seasons in each case (his rookie year wasn't prolific, but it was excellent by rookie standards). A player without any drive would probably take the foot off the gas if the club isn't doing well.
Now one could argue that those two indicators could be driven by a desire to get paid, and he could change those habits once he has financial security. And that could be true. Which is why we once again have to come back to the personnel department and their read on his character. If they worry that he's the type of person to check out when he gets a payday, then by all means they should pass. However, if that's not their read on him at all, then I don't think you can afford to pass on elite talent out of fear of what may happen (providing that the price to acquire said talent is low enough that it still adds net value).
And it's true that past health is not a guarantee of future health. However, the best indicator of durability is past durability and the best indicator of injury is past fragility. Mack could become a Packer and break his leg on his first day of practice. It's an odds game. His background suggests that his odds of staying healthy are as good as any player's.
A note on his production-- we all know that sack numbers can be a bit capricious. Mack's sack totals have come down from 15 in 2015, to 11 in 2016, to 10.5 in 2017. But according to PFF, his total pressures in 2015 and 2017 were very similar (79 vs. 76). Take that for what it's worth-- not everyone loves PFF and "pressure" can by hard to agree on. But suffice it to say, I don't think there's been any sort of drop-off.