This makes no sense for SoftBank.
They are paying a lot of money ($32 billion) for gross income of around $1.2billion, and profits of about 1/2 of that. Based on that, it will take 30-40 years for the investment to pay SoftBank off, based on ARMs revenue alone.
Something doesn't add up. On one hand, they're saying that they don't plan to change their business model, or even how their R&D is done in the next years, but on the other hand - they will need to come up with something really amazing if they want to recover this investment.
You can buy a lot of other things for $32 billion that would be a far safer bet.