ARRI is not Canon's biggest obstacle. CANON is Canon's biggest obstacle. There is still a huge demand for film and Canon don't make any film cameras. it's that simple.
The future isn't film, though...and digital is rapidly gaining ground on film. Canon would have to shift resources from digital cinema to film in order to build up a product line and gain a presence...in a market that will decline in the long term. Not really a wise move. I think Canon has it right, building up a presence in digital cinema.