OptoIsolator Reliability?
The best guide to component reliability I've ever come across is MIL HDBK 217 F. It details the reliability of components from the analysis of thousands of pieces of faulty/failed equipment gathered over many, many years.

For a simple, fixed value (&lt; 1 Mohm) film resistor (section 9.2), the basic failure rate \$\lambda_b\$ is 0.001 failures per million hours at stress rate 0.5 and operation at 20 degC.

For an opto isolator (section 6.11) of the type with a photo diode the failure rate is 0.0025 times per million hours and, for a phototransistor it is 0.013 times per million hours.

So MIL HDBK 217F is suggesting that an opto might be about ten times more likely to fail than a simple resistor but it's still in the vicinity of once every 11,000 years!

Compare this with a diode (section 6.1) used for power rectification - it has a failure rate of 0.069 times per million hours - several times worse than an opto-isolator. 

How many of these are you using in your design?