How do you know when to retire DVD burner?

I have a NEC 3500 that still burns DVD’s but lately I’m getting bad burns on and off. Disks that have PO falures. Some disks burn fine and some with PO failures. I’m using TY 8x media only and can’t figure out if it’s a bad batch of media or the drive is dying. Normally if the drive fails to work you replace it. Can the laser get weak slowly or something else that dies slowly or do you consider the drive good just because it is still burning DVD’s?

The laser can get weak over time, the firmware is out of date for modern mid codes, etc. If you like the drive enough pop it open and clean the laser optics and see if it works OK after that. They can get real dirty after some years of use and work poorly till cleaned if the rest of the system is still good.
The 3500 was a great burner in it’s day, excellent quality though a bit slow compared to others at the same speed.