I'm speaking here as an end user of gcc, who might want e.g. to make a nice code formatting plugin which has to parse the AST to work properly. For a long time, Stallman's demand was that gcc's codebase be as difficult, impenetrable, and non-modular as possible, to prevent companies from bolting a closed-source frontend to the backend, and he specifically opposed exporting the AST, which makes a whole bunch of useful programming tools difficult or impossible.

Whatever his motivations were, I don't see a practical difference between "making the code deliberately bad to prevent a user from modifying it" and something like Tivoization enforced by code signing. Either way, I as a gcc user can't modify the code if I find it unfit for purpose.