The real reason we needed Marie in the finale.
The final battle nerfed Homelander into the ground. He is faster than Atrain, is shown lazering supes in half, and durable enough that Butcher’s tentacles had little to no effect on him in S5E1. We needed Marie there as a support character slowing Homelander and healing the Boys as they were getting Ripped apart. This is how you can show the payoff from her powers without just having her bust open Homelander like a blood balloon.
And if you wanted extra emotional payoff from the Gen V crew you could have had Jordan there and Homelander kills him while being held down to be Kimiko blasted and gives Marie that last adrenaline rush to hold him down enough for Butcher and Ryan to keep him in place for the blast.
Marie would have been the catalyst for the writers to go all out showing this final brutal power display from Homelander. But instead we conveniently get lasers and punches that don’t have even close to the power we have seen from Homelander even just a few episodes ago.