[rzwitserloot]

You're still maintaining it. In the sense that _if_ it fails in the future (which is admittedly unlikely, given that it hasn't done so in the past 5 years. But as stock investment services say: Past results are no guarantee for the future!) - you must fix it, and fairly quickly. So you need to maintain the toolchain and be at least sufficiently familiar with it that you or somebody in the team can hop in and fix things if needed.

That, on its own, is a cost, and that cost can warrant a refactor. To go for an extreme:

It's written in COBOL, it is the last cobol code in your entire code base, and the only engineer with any experience in cobol is about to retire.

If that happens, I'd refactor it. If you don't, and 2 years hence the customer calls in a panic that the product they are still paying for isn't working, you're really screwed. I doubt the customer is willing to wait for you to __learn COBOL first__.

But, merely 'the code is not at all written with modern practices and our current team's preferred style and such in mind; it wouldn't pass code review right now?' - yeah, don't refactor that, absolutely.