Many people are saying that Lewis will stay in Mclaren because how of much Mclaren/Ron Dennis have invested in his career.
I think Lewis has already "paid back" what Mclaren have invested in him by winning for Mclaren their first championship in almost 10 years. So from now on, it will be purely business and the moment Mclaren stop producing a championship winning car, Lewis will leave because he is a champion and he is very marketable.