Do CPLD devices lose non-volatile memory over long periods of time?
The Xilinx data sheet XC9572XL datasheet states that the programming should be good for 20 years.  I would expect even more if the temperature is moderate.



I would look somewhere else for problems.  Electrolytic caps are the usual suspects for deteriorating with age. Make sure the power supplies for the CPLD are good - measure with a scope as well as a meter as there may be a large ripple if the caps in the PSU have degraded.