Why is that good news, when it also implies that in the vast majority of worlds/branches, you lost the lottery? It only makes sense if, after learning that you won, you no longer care about the other copies of you that lost, but I think that kind of mind design is simply irrational, because it leads to time inconsistency.

I want to be careful to distinguish Many-Worlds (MW) branches from theoretical possibilities (with respect to my best theory). Events in MW-branches actually happen. Theoretical possibilities, however, may not. (I say this to clarify my position, which I know differs from yours. I am not here justifying these claims.)

My thought experiment was supposed to be about theoretical possibility, not about what happens in some MW-branches but not others.

But I'll recast the situation in terms MW-branches, because this is analogous to your scenario in your link. All of the MW-branches very probably exist, and I agree that I ought to care about them without regard to which one "I" am or will be subjectively experiencing.

So, if learning that I played and won the lottery in "my" MW-branch doesn't significantly change my expectation of the measures of MW-branches in which I play or win, then it is neither good news nor bad news.

However, as wnoise points out, some theoretical possibilities may happen in practically no MW-branches.

This brings us to theoretical possibilities. What are my expected measures of MW-branches in which I play and in which I win? If I learn news N that revises my expected measures in the right way, so that the total utility of all branches is greater, then N is good news. This is the kind of news that I was talking about, news that changes my expectations of which of the various theoretical possibilities are in fact realized.