You are looking at this for the wrong angle. You have inherited working code generating revenue but in state which makes it hard to develop new features and ma

You are looking at this for the wrong angle. You have inherited working code generating revenue but in state which makes it hard to develop new features and manage productively. As you say that the roadmap is agressive and management has no understanding of the situation, you have already established what you have to do: explain to management what makes development difficult (avoid statements like this is the worst and focus on what needs to be done to establish best practices and identify where the quick wins to gain development velocity are - more expertise and less judgement is always a good idea). Then you propose a realistic roadmap and start making the changes that need to be done.