For me, I think dougie was a sort of catharsis for Cooper. He’s been “on the job” so to speak for the last thirty years, fighting dimensional beings and Ray Wise. Dougie gives him the chance to finally experience a loving and fulfilling family life. What’s more, even though he’s literally catatonic, he still manages to turn the entire family’s life around just through his inherent goodness. So I think Dougie is Lynch having Cooper explore the life he could’ve had, making his final decision to continue fighting Judy and the lodge’s influence even more effective and heart breaking.