You need to consider NSH's window to compete for the Cup. They have a good shot to win now and the low AAV was essential to that effort. Same holds true w/ Tanev in PIT. Both teams are in win-now mode and consider cap space to be more important than term. I actually love this deal for NSH. Poile gets an A+ in my opinion.
I don't think Nashville having a shorter window has anything to do with the length of this contract. If anything, Nashville paid additional AAV to get more term. Sissons is a very good depth player, but he is getting more than most of his comparables in terms of AAV. Copp has similar production, role and age, with similar impact metrics on ice, and he got over $600k less in an arbitration award over 2 years. Lehokonen, Armia, and Sundqvist are RFAs who got a lower AAV and term with similar production.
This isn't to say that he isn't potentially better down the road than anyone in that group, just that he didn't have any more leverage than they did, so the additional term wasn't to keep the AAV down, it's a bet by Nashville on his future value.
The issue I have with betting on SIssons future value is that it largely assumes that his deal will look better relative to the cap as time goes on. That may be true to some degree, however we have already seen that this tier of player hasn't had their RFA deals inflate at the same rate as the cap, and that is because teams are starting to allocate more and more of their cap to their star players. So star player salaries have been rising a little faster than the rate of the cap, while second tier players salaries (on average) have been rising slower. Looking for instance at some comparables from a few years ago, Roussel, Wingels signed in the summer of 2014. They signed for RFA deals between the ages of 24 and 26, and Roussel and Wingels signed for $2M and $2.475M respectively. Also, they are great examples of how those deals don't necessarily age well. Wingels was out of the league before he was the age Sissons will be at the end of his deal, and Roussell is still a solid 30 point forward with strong analytics against bottom 6 opponents, and he was signed as a UFA for $3M which was considered an overpayment.
The deal could work out fine, it's not as big a risk as some, I just don't see the big win scenario to make the risk worthwhile, and it's pretty obvious the Preds didn't have to give the term, it appears that they chose to do so. So it just seems a bit weird.