Faulty formulation: Committing a major sin automatically and immediately removes every sinner from Islam.
Correct formulation: Not every sin is equated with disbelief. A monotheistic believer who commits a grave sin may deserve warning and accountability; the final outcome remains with Allah's will, mercy and justice.
Application: Minimizing the sin is wrong, but so is declaring every sinner an unbeliever without the required grounds. The prohibition of an act, its gravity and a judgment of disbelief are separate questions.