I tried to hack on g++ as part of project years ago, but really struggled to make any contribution. The codebase is indeed monstrously complex, but the core development team is also very protective. A secondary effect of the growing complexity in an open source project is the decreasing willingness for the core team to bring in new people because of the time/effort cost involved and potential risks. If the team naturally atrophies over time, you end up with increasing stagnation.