He's hinted that he could release the sources if the Steam/Itch sales make enough money that he could stop worrying about the stability of his income in the long term. The worry was that if DF got open-sourced and forked, people would give more money to the forked, polished version as opposed to the one version Toady is working on, but he wouldn't mind if he had enough money in the long term.
catacombs|5 years ago
The saddest part of Tarn's decision to put the game on Steam is to make enough money to pay off medical bills for his brother.