In my humble opinion, a lot of projects go quietly bad when they experience some sort of new requirement that gets underestimated in terms of its architectural impact by project management. At such time senior Devs have either already moved on, or have their eye off the ball such that new features get incorporated without the necessary architectural support. These inflection points can themselves introduce complexity but often become the gateway for all sorts of subsequent small things that explode in size. In short, don't miss architecture moments
The other thing that happens, and really dooms a project is when the senior people leave. Eventually you end up with a team that doesn't really understand the code.
This leads them to just tack on features while changing as little as possible. This will grow into something truly unmaintainable, virtually guaranteeing no competent work will be done on the project again.
