you seem to be the type of person to be content that if it runs, its probably ok. nothing wrong with that as you are ok if a breakdown occurs. if this is the case, why bother with a change? sparking fine so leave it as is.
if you were the type of person to want to be proactive in regards to breakdowns, then you would have changed them earlier as a preventative measure.
there is nothing wrong with getting every ounce of life from the wires. you just have to be prepared for the breakdowns and inconvenience.
i say be adventurous and see how long those wires will last.