Well they have to either sell them in a market or use them in a way to have a greater income than outcome. Since I don't believe in the latter, I'd bet that there're people who buy them. Still, at the moment each block gives a tiny amount of coins, but it's too expensive. I can't really speculate anything farer.

I remember I had created this thread back when I had more, similar queries: What's the point of mining testnet bitcoins?