> the profit appears to be un-earned
The ability to make profit off of investment is fundamental to capitalism. Do you think that it's unethical for your 401k to accumulate compound interest? Should you have to do some work for the companies your 401k holds shares in, in order for your fund to make a trade?
So far Chris Chen has put in a ton of work and only now is realizing the gains from it. His risk and work are only now paying off. This is equally as egregious as your employer hiring you to write C++ code, and reaping the benefits of your code long after you have written it.
I don't see how competition is closed. The only competition I see suppressed is that of the "order a painting from a website" category, because Instapainting has the superior network. In time, this could potentially get so huge as to be a violation of anti-trust laws, but I don't see that happening. Competition between painters is untouched. Other applications remain. If one wants a higher or lower quality painting, the buyer would want to use existing networks instead of this service. If someone wants an abstract painting, or a sculpture, they have to use a service that is less commoditized, such as word-of-mouth or an art company (interesting aside, North Korea fulfills many sculpture orders for national monuments internationally).
Within the realm of medium-quality portraits, if Instapainting secures a monopoly, then yes, competition will be closed. However, this applies to almost anything that can be commoditized. In American/Canadian history, 19th century painters decried a commoditization of their work; they couldn't get commissions for impressionist or surreal work. Just portraits.
I wrote this blob of text because I naturally want to defend developers who expose details about their businesses to HN. I don't want stories of how they built their businesses to be suppressed or discouraged. Especially since I see nothing unethical in this product; on the contrary, Chris Chen is giving these artists opportunity. They don't have to spend unpaid hours on Etsy or eBay, they can just focus on their work. Customers also get the opportunity to get a hand-painted portrait.