Is it usable because it's... usable, or is it usable because it's barebones? It can be hard to tell the difference when it's pretty far from feature parity with the competition.

FWIW, I don't think cornering the non-professional developer market will lead to market dominance. GameMaker is even simpler, and even if it were open source, hobby developers will not be competing technically against thousands of full time, professional engineers working on an engine.