Merriman did a lot of heavy lifting in Part I
Butler was the star and helped carry Part I, of course. But Merriman was very important too. Although he was the thief, his character was more complex and showed more redeeming qualities than Butler’s character did. Audiences might’ve left Part I feeling more drawn to Merriman’s character . Donny in Part I was deliberately the least compelling character (to make the twist work)
I’m sure they will introduce some new characters in Part 2. But you have to wonder if the absence of Merriman might be a handicap