Thing is, if they were dead set on the trio being iredeemable then Grogar was a hinderance. You could argue that he shouldn’t have been introduced at all, but with the way they wrote the premiere they had three options:
A. The Trio are jobbed out early on in the finale by the heroes in order for Grogar to pull a Thanos and “do it himself”. Which would make them look bad.
B. The Trio are bitched out by Grogar early on and he takes over as Big Bad, in which case why did we bother focusing on the villain trio so much?
C. They get rid of Grogar and the trio are the big bads of the finale.
They chose option C. The finale was always going to be bloated because it’s the triumphant ending of a show with a billion characters. Grogar in a big role would add yet ANOTHER thing to deal with. Another thing to explain and exposit about.