Hey there! That's a really smart question - you're thinking like a true data-driven plant manager. I've been looking into this exact dilemma, and here's what you should track over the next 6 months to make an objective decision:

First, on the technical side, you'll want to monitor:

1. **System Downtime**: Track every minute your S7-300 causes production stops. This includes both planned maintenance and unexpected failures.

2. **Failure Rates**: Log every hardware failure - CPU, I/O modules, power supplies. Siemens is actually phasing out S7-300, so spare parts are getting scarce and expensive.

3. **Maintenance Hours**: Record how much time your technicians spend troubleshooting, repairing, and maintaining the old system.

4. **Compatibility Issues**: Note any problems integrating with newer equipment or software. The S7-300 uses older STEP 7 Classic, while newer systems use TIA Portal.

For financial metrics, track these carefully:

1. **Total Cost of Ownership (TCO)**: This is crucial! Calculate all costs - not just hardware, but maintenance, spare parts (which are getting pricier), technician training, and energy consumption.

2. **Downtime Costs**: Convert your downtime minutes into actual dollars lost in production. This often reveals the true cost of keeping old equipment.

3. **ROI Projections**: Siemens offers an S7-300 Upgrade Check tool that can help estimate migration costs to newer systems like S7-1500. Compare this against your current TCO.

4. **Future Scalability Costs**: Consider what it would cost to expand or modify your current system versus a modern replacement.

Here's my advice: Create a simple spreadsheet and track these metrics religiously for 6 months. The data will tell you the story - whether the emotional attachment to your familiar S7-300 is worth the increasing costs and risks. Modern systems like S7-1500 offer better reliability, IoT integration, and long-term support that could save you money and headaches down the road.