It allows for the Vegas Golden Knights to address the roster further
Indeed, $5.5 million AAV was certainly doable by Kelly McCrimmon's standards. All Vegas had to do was make some moves, such as moving Robin Lehner to LTIR, to make it happen. That way, it would've given them the necessary salary cap room to work.
However, there are other deals that need to be focused on. For example, Jack Eichel has two years on his current deal. He'll be due for another contract and is younger and more versatile. With the Vegas Golden Knights needing to address Eichel's situation, that will cost them plenty of salaray cap room.
That's also true if they want to sign a superstar like Mitch Marner or Leon Draisaitl. The Golden Knights are one piece from ultimately shooting themselves up to being Stanley Cup favorites with such a move. Therefore, creating the necessary space is key.
Yes, Jonathan Marchessault's deal with Nashville might look miniscule in five years compared to where the market looks now. However, $5.5 million AAV makes a notable difference when creating room for big-time stars. Therefore, it makes sense for the Golden Knights to pass on making such a deal.