The transmission and distribution electric lines would remain a monopoly. The non-utility companies see a huge payoff in deregulation. Regulated utilities aren't welcoming deregulation with cries of joy. Everyone likes deregulation if it will bring them lower prices. No one likes deregulation if it brings their business more competition.
Regulated utilities have another problem.
They built all these gold-plated generating plants back when it made perfect economic sense to do so. But in a deregulated market, those plants turn into white elephants. The non-utility companies can produce electricity cheaper. Regulated utilities realize that deregulation is inevitable. They simply want to put it off for as long as possible.