Optoisolator Long term Reliability at HVDC
If reliability is a problem get a part with an MBTF listed in the datasheet, that means that the supplier built it with longevity in mind and tested it. Generally ic manufacturers must live up to the specs in the datasheet (read the legalese printed on most suppliers datasheets). Once you have a part that is guaranteed to work for a long time, make sure it doesn't get damaged by running it outside of spec, overvoltage, ESD and that it has the proper soldering time at the PCB assembly house. If your really interested in reliability, then derate it and make sure that the parts specs are over-preforming in every way. LED's generally have a lifetime of 100k hours, I'm not saying that this applies to optoisolators but the physics are the same. Maybe you could build the design so its running condition runs with the LED off.