The problem I'm anticipating isn't so much "the codebase grows beyond the agent-system's comprehension" so much as "the agent-system doesn't care about good architecture" (at least unless it's explicitly directed to). So the codebase grows beyond the codebase's natural size when things are redundantly rewritten and stuffed into inappropriate places, or ill-fitting architectural patterns are aped.

Don't "vibe code". If you don't know what architecture the LLM is producing, you will produce slop.