why would someone senior even want to join this team? Especially someone senior enough to fix this. The productivity is horrible and there's no kudos for fixing something that's lived for 12 years like this.
Theoretically a company that's making $20m/year on this can afford to make it worth someone's while to come in and fix it. The problem isn't finding someone who will do it, it's that the company assumes they can continue to get by indefinitely on paying too little.
Mixing skill levels in a team is healthy.