>yesterday we only had a lower bound on AlphaGo's strength; today we also have an upper bound
I think it's premature, establishing bounds with good confidence interval requires tens or hundreds of games. Specifically, 3:2 result would be really inconclusive.
The first data point tells you more about bounds than any individual subsequent data point. Knowing that a human can defeat AlphaGo is enormously important.
To use an analogy, having confirmation of contact by even a single alien species would be hugely important, way more so than exactly nailing down the number of alien species. Knowing that something is even possible is oftentimes the most important aspect that needs to be ascertained, and contact (or a win, in Lee's case) does that unequivocally.
At the time chess was "the" problem, humans could beat computer too from time to time. Now the chess is solved. Same will likely happen to go in the years to come. But the point of this was not the go itself. It was the demonstration of potential capabilities of neural network and other machine learning algorithms.
Chess isn't solved. It's just that computers have gotten sufficiently better at it than humans that humans don't have a chance.
If Chess were truly solved, then you wouldn't be able to make a new AI program that could do better than even odds against the existing ones. But that's not the case, and incremental advancements in Chess-playing programs are made all the time. There are even tournaments where Chess programs play each other. If Chess were solved, such a thing wouldn't make any sense, just like how there are no Tic-Tac-Toe tournaments because that game is solved.
Even that isn't strong enough. The game is determinstic.
If chess were _solved_ we'd know a strategy to allow one of the players (likely white) to always win, or for either of them to always force a draw. (and we'd know which of these strategies were possible for chess).
Consider, say there is a first move white could choose such that no matter what moves black makes, white will win. Then the first would be true. Instead consider, that there is no such move, and any first move has choices where either could win-- if some of those are ones which would force a black win, then the first is again true but for black. Otherwise, a draw can always be forced. These are the only possible outcomes for a solved game of chess.
I think it's premature, establishing bounds with good confidence interval requires tens or hundreds of games. Specifically, 3:2 result would be really inconclusive.