Pro: Increased Trade Value
If the Cavaliers continue to see flaws and issues with the Evan Mobley/Jarrett Allen duo, they could decide to move Allen in the future.
Now, if the Cavaliers decide to move Allen, he will have greater value in a trade. If a team is looking to trade for Allen, they will be more inclined to do so if he has 3+ years left on his contract. With Allen's old deal, teams would be nervous that they would be trading for a rental, who would leave for his next team when his deal expired.
At roughly $30 million per season, matching salaries is a fairly easy task for the Cavaliers. Either they can package Allen with other players to chase a max-level player, or they can swap Allen for a collection of players in return to add depth and fill needs. As the NBA salaries continue to grow, Allen's cap figure will be more easily transferred if needed.