I recognize that it's not just ARM's business model that poses a threat; the threat stems from a combination of ARM's business model, its current technical advantage (in energy efficiency), and trends in computing, with a shift away from PCs (and the Wintel standard) to connected devices running Android and iOS. To your point, the shift away from the Wintel standard has significantly reduced the barriers (i.e., backward compatibility) to the adoption of the ARM architecture for OEMs.
(Note, ARM's recent "rapid rise" in the processor market is illustrated, in part, by the company's stock market performance, with a market cap that has grown about 5x over the last two years.)