Ugh, seen this pattern happen many times at smallish companies. The O.G. codebase was written by a single person. It did one or two things very well, with few bugs, had consistent filenames, variable names, APIs, formatting, and so on, it had a single, pure vision throughout the code, and didn't have all those problems that happen when multiple people reason differently about the same code.
Then, eventually, the company gets bigger and the product needs to grow beyond the capability of one person, so a team starts developing on the codebase. That's where the bugs start pouring in, where everything becomes less consistent, where different design patterns are used all over the place, and where the "single clear vision" gets cast aside. A lot of problems that get blamed on "technical debt" and "brittle code" are often just plain coordination and consistency problems that are happening because multiple people can't share a single consciousness.
Then, eventually, the company gets bigger and the product needs to grow beyond the capability of one person, so a team starts developing on the codebase. That's where the bugs start pouring in, where everything becomes less consistent, where different design patterns are used all over the place, and where the "single clear vision" gets cast aside. A lot of problems that get blamed on "technical debt" and "brittle code" are often just plain coordination and consistency problems that are happening because multiple people can't share a single consciousness.