It's an expensive problem, especially if it's a system that's being used all across the airport by regular staff.

You need to train thousands of employees to use the new software, you need to have one person using the old software as a backup, while the other uses the new software, often while surrounded by hundreds of often angry customers.

And if something goes wrong, which it invaribly does (even if it's user error or someone snagging a cable), shit can get very expensive. Small delays, add up to larger delays, and cascade through the entire system. Delayed flights, tens of thousands of euros in costs, hotels for thousands of passengers, missed flights, missed meetings, damages, lawsuits, penalties for missed landing/take-off slots, missed time windows for certain cities which don't allow flights after a certain time, etc. And often you discover legacy stuff while you're upgrading that needs fixing, stuff that no one knows how to replace anymore or is physically hard to access.

Sometimes it is genuinely better to leave it. COBOL is 60 years old. There's still plenty of stuff running on it, exactly because it's often too expensive and too risky to replace.