Comment by hnthrow0287345

If there's a legit, measurable performance or data integrity problem, start with that. If most of your production bugs come from a specific module or service, document it.

If it is only technical debt that is hard to understand or maintain, but otherwise works, you're going to have a tougher time of building a case unless you build a second, better version and show the differences. But you could collect other opinions and present that.

Ultimately you have to convince them to spend the time (aka money) on it and do it without making things worse and that is easiest to do with metrics instead of opinions