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.
Plus the size of project that an LLM can help maintain keeps growing. I actually think that size may no longer have any realistic limits at all now: the tricks Claude Code uses today with grep and sub-agents mean there's no longer a realistic upper limit to how much code it can help manage, even with Opus's relatively small (by today's standards) 200,000 token limit.