retrocomputing

Well it technically works...

Nope. There's your problem. You don't start with any version of "it works." You begin with "here's how the old system is costing us money, and here's how much more money the new system will increase profits." Tech debt is the catchphrase of the moment, and you can describe it in financial terms executives understand. Tech debt charges interest, and the interes of delaying payments into infrastructure will outpace the money saved by a significant margin. That's why spending the money now will decrease friction, reduce churn, and streamline onboarding of new accounts. This is what our competitors are doing, and this is how we can do it better.

That's the pitch. If your argument is "it works, but we can do better," that makes it sound like you're putting extra padding in the first class seats. The real answer is it works because IT is working in a constant state of panic and the job of 10 engineers is being done by two interns and a roll of duct tape. To an executive, that sounds like efficiency, because they don't give a shit about you. Number goes up, that's their focus.