Lee is widely considered the greatest player of the last decade, winning 18 world titles, but has been surpassed in the last few years by younger players, with this unofficial ELO rating system ranking him at No.4: http://www.goratings.org/. Go is really a young man's game right now.
Even if he is now, there is a probability that people will learn how to beat AlphaGo.
In a sense this is unfair: alphaGo was trained with a lot of human data, but AFAIK Lee Sedol is playing AI for the first time.
The probability that any human player will learn to beat state of the art chess engines is zero (at least until we have humans with biologically or electronically augmented brains). There was a very small time window in which an expert player could beat chess engines by 'adapting their style'. Do you have reason to believe Go will be any different?