It was luck that a viable non-graphics application like deep learning existed which was well-suited to the architecture NVIDIA already had on hand. I certainly don't mean to diminish the work NVIDIA did to build their CUDA ecosystem, but without the benefit of hindsight I think it would have been very plausible that GPU architectures would not have been amenable to any use cases that would end up dwarfing graphics itself. There are plenty of architectures in the history of computing which never found a killer application, let alone three or four.