It entirely depends on the description language. In Python, the opposite is true for basically the reason you gave: an asymptotically positive constant fraction

It entirely depends on the description language. In Python, the opposite is true for basically the reason you gave: an asymptotically positive constant fraction of valid Python programs will begin with code searching for a contradiction to ZFC followed by an exit statement. In the model where Turing machines are modeled by random graphs with out-degree two, it is no longer necessarily the case that the start state component is exactly some fixed undecidable subgraph; i.e. as more states are added the probability of finding the start state inside a particular induced subgraph shrinks to zero.