If there are more sales, but at lower prices, and it balances out, it only hurts you if you thought you would get a bigger share if people hadn't spent money on other games.

If it doesn't balance out, if sales have decreased spending, then fine, maybe there's a problem. But I doubt that, give that Steam's sale frequency increased after they started.

So how can I tell that he's not just complaining because he wishes he'd gotten a larger share, that he feels he is losing out because people are spreading the money around more?