[GMontag]

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It can stay the same. It can also decrease. Or are you saying that once you lose your fortune you can *never* get it back?

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You're right, it/you can. That's not the issue. We're talking averages here.

Take a list of all the fortune 500 companies. Let's say that on average, X% of those companies will not be fortune 500 companies in Y years. If we increase Y, on average, X will increase.

This is a logical truth.

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No it is not. This isn't hard. It is only true if the decay rate is constant. Since you have neither specified the behavior of the decay rate, nor can you infer anything about the behavior of the decay rate from the single set of data ITT, your conclusion is not valid.

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AlexM made the assertion that 50% left after 20 years implies 25% left after 40 years

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He used those numbers with the caveat that he didn't mean exactly those. If he simply said that the number would be significantly smaller then 50% in 40 years as opposed to 20 he would be undeniably correct.
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It could just as easily still be 50% or 40% or (maybe not quite as easily)even 60%.

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With the possible exception of 40%, no, no it couldn't.

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Jesus christ, this isn't hard. All three of those are possible because you don't know anything about the behavior of the decay rate.