I feel secretly rewriting software from scratch is rarely the right solution. Especially if management has already decided thats not the direction they want to

I feel secretly rewriting software from scratch is rarely the right solution. Especially if management has already decided thats not the direction they want to take. Your job as an employee is to make your boss happy. If you don't have political buy in from your boss then he/she isn't going to be impressed that you have been spending all this time on this project instead of spending that time on whatever it is that is important to them. Even if its overtime. If you disagree with your boss that radically, then you should find a new boss because you won't be successful in your current position. And also because you won't ever be happy. Find a boss who you get along with. And be happy.