Hey, that's a really smart question! Dealing with 15-year-old automation systems that manufacturers have abandoned is like walking a tightrope between two expensive cliffs. On one side, you've got inventory costs eating into your budget, and on the other, production downtime that could cost you way more.

Here's how I'd approach it: First, do a criticality analysis - figure out which parts are mission-critical. If a specific PLC card fails and shuts down your whole production line for days, that's a high-risk item worth stocking. But if it's something that can be temporarily bypassed or has a workaround, maybe you can risk not having it on hand.

Next, look at failure rates and lead times. Some parts fail more often than others, and with legacy systems, replacement parts might take weeks or months to source from third-party suppliers or refurbishers. For high-failure-rate parts with long lead times, it makes sense to keep spares.

Also consider alternative sourcing options. Can you find compatible parts from other manufacturers? Are there companies that specialize in refurbishing legacy automation components? Sometimes partnering with a third-party maintenance provider who already stocks these parts can be more cost-effective than maintaining your own inventory.

Finally, don't forget about the bigger picture - maybe it's time to start planning for system replacement. The cost of maintaining obsolete systems often catches up with companies eventually. A phased replacement strategy might be the smartest long-term play.

What specific types of automation systems are you dealing with? That might help narrow down some more targeted strategies!