When AlphaGo does lose, it seems to happen when outright bugs cause it to make moves that are readily recognizable as mistakes. It doesn't seem to happen because it's not quite "smart" enough, or because its underlying algorithms are fundamentally flawed.

That's a big difference. Bugs can be identified and fixed. By the time AlphaGo faces another top professional (Ke Jie?) we can safely assume that whatever went wrong in Game 4 won't happen again.

Consider how much stronger the system has become in the few months since the match against Fan Hui. Another advance like that will place it far beyond the reach of anything humans will ever be able to compete with.