I really don’t think one can overstate the importance of Luther in this movie—not for the audience, not for the franchise, and especially not for Ethan.
For the audience he comes in after a long dark spell. First-time viewers wouldn’t know if they can trust him yet; but he brings humor and warmth and new interest. Krieger lends supporting humor and a secondary point of fresh interest; but he is clearly untrustworthy. It’s Luther who gives the audience someone to cling to: a hope for reliability in a film world where, aside from Ethan, everyone left standing is either a confirmed antagonist or highly suspicious. Without Luther, it’s hard to imagine the film sustaining the shift from darkness into the tense but decidedly enjoyable heist. And without him sitting with Ethan at the end, even when Ethan is successful there would be no image to leave the audience but one of desolate and solitary heroism—and who knows if the series could ever have clawed its way from such isolation to the found-family that is now at its heart.
And as for Ethan—well. Without Luther, he walks out of this movie having learned that loved ones are people you either lose or are betrayed by. Ethan is resilient and he is good, but it’s not hard to imagine that without Luther this might have been too much. At worst he might have become another Ambrose, or another Lane. Perhaps he might have continued working for the IMF, but with all the jaded callousness and inability to form lasting human connections of your typical Bond-esque hero. Or perhaps he just. Breaks.
But Ethan takes a shot in the dark—digs up the nearest disavowed agent with skill-set he requires—and it’s Luther. And Ethan chooses to trust him, and Luther proves trustworthy. And Luther stays.
So the day after losing everything, Ethan meets the man who will become his best friend. Even as his agency betrays him, he finds through another man they rejected that trust can still be well-founded. And as he steps forward to rebuild his life, Luther walks alongside him. He is not alone.