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I am however, earning this money primarily through one hobby site
be it a change to Google's business model or to competition.
I think that running more sites actually increases your risk of income loss.
I'm still working towards making a living from my sites (though I don't have a job) and would not be happy to have all my income from one site.
I'm looking to re-invest
You're right. I think Google started with just one site and they've been diversifying ever since. Although I could argue that all the sites they've launched don't make money and pull valuable resources away from search...