Marchessault is undersized
Being 33 years old is one part of the equation. When you request five years for your next deal, it makes it tough for a team to prepare for the future. That's especially true when that player is physically lacking. That's the case with Marchessault, who only stands at 5'9".
Being at that height limits a hockey player and what they can do. For example, it leaves such players vulnerable defensively, limiting how they check and move. Granted, the "Golden Misfit" weighs 185 lbs., so that won't be a problem now. However, when he gets closer to 39 years old, his skills will deteriorate.
That's also true offensively, where taking the puck away from him is much easier. Scoring 42 goals in a single season is an impressive feat, especially at 33 years old. It'll be tougher to duplicate that season with a small stature. It's easier for opponents to take the puck away since he can't use his frame to turn them away.
The Golden Knights pride themselves on playing bigger than their opponents. That's why they were the second-tallest team last season (74.1 average inches) and the heaviest team (207.9 average lbs.). It's reasonable that losing a smaller piece helps Vegas become bigger.