i think the point to start rewriting is when nobody still on the team can maintain the product. i recently got pretty close. i had been researching something for months (speeding up a login), but to no avail. so to avoid looking even more incompetent i said, you know what, an SSO app should be relatively simple to create. just slap some strings together and youre done. it would be quicker than banging my head against the wall for another three months. but somebody on another team still had knowledge and this ultimately gave us the idea to fix it.