The payoff from AGI may be incalculable, but it isn't infinite both in itself and in Altman's ability to enjoy the rewards it promises. Once the value becomes finite, a whole heap of risk-reward logic kicks in that the Wager wants to sweep under the rug.

As a concrete example, following this Altman's wager would result in Altman giving all his wealth to the first beggar on the street who mumbles that he might be able to run an AGI project - the possibility that the beggar can achieve that is, technically speaking, nonzero. Multiply that by infinity and you have a great expected return (infinite, in fact). However, practically speaking, the risk will overwhelm the large-but-finite payoff.

Infinity is bigger than people think :P