Have you thought about taking some of the income and using it to hire other developers to increase the number of apps that you've got being developed at once? It seems like you know what works and doesn't work on the app store, so it would probably make sense to start trying to make your efforts scale up.
things that i would add:
- consider just sitting on (some/most of) the money for a while. you don't need to make a big hasty plan and execute just because you have some cash to spend right now. thats not to say do nothing. just don't throw all your resources at something immediately just because you have the resources to burn.
- diversify. as you said, the app store won't be forever. try looking into other things that are either just becoming viable for development, or things that are going to be longer-lasting.
- pad your account. no matter what you do, there will be dry times and hard times. make sure you're good to go for that. stash some money away while you have it.
Since I tend to agree with that assessment, I couldn't recommend doubling down on building apps.
There may be other good reasons to consider that, such as moving in another direction (consultancy, which raises the interesting question: is there a such thing as iPhone app consultancy?), but not for that reason alone.
Edit: Google says that there is such a thing. It strikes me as likely that would be more stable long term, and, potentially, even more lucrative.
However, there is still a metric shit-ton of money to be made on the app store. I don't see people leaving it in droves immediately. He knows how to make money in this environment right now, so in my opinion he should spend at least some of the money he wants to invest on something he already knows how to do well.
I just don't see the wisdom in avoiding a marketplace you already know how to make money on; even if it has a chance of going away. Max has already proven that he is able to identify a market and take advantage of it once, who is to say he couldn't take advantage of the "next big thing"?
Then with the last %25, use that to invest in more appstore / development projects.
Why not do more of it? Scale up?