Human intelligence must be deterministic, any other conclusion is equivalent to the claim that there is some sort of "soul" for lack of better term. If human intelligence is deterministic, then it can be written in software.
Thus, if we continue to strive to design/create/invent such, it is inevitable that eventually it must happen. Failures to date can be attributed to various factors, but the gist is that we haven't yet identified the principles of intelligent software.
My guess is that we need less than 5 million years further development time even in a worst-case scenario. With luck and proper investment, we can get it down well below the 1 million year mark.