While starting out, knowing what not to do and precisely why is nearly as important as knowing what worked. In the case of good codebases and bad codebases though, you still need to be careful not to cargo-cult wholesale the architecture that worked before, and conversely not to do everything different from the last horror you worked in. View it as a learning opportunity as you debug: some things they will have gotten right, sounds like many things have been gotten wrong, but the process of reasoning them apart is still valuable.

All that being said, certainly do not hesistate to look around if you feel like you aren't growing as fast as you could be. Life is short and it's a sellers market for engineer labor in most places I have seen.