Modern plugs are better I suppose, but 31K miles is a heck of a long time, that could be 10 years in some cases..!!
Depends on the condition of the plug. You can get a carbon build up inside the plug if the engine has a slight burn issue, the spark then tracks on the carbon and can cause more issues, sometimes cracking the ceramic so the spark is not concentrated where it should be. I have seen old spark plugs that look like a knarled chunk of metal where the spark electrode should be and very little ceramic left to speak of. As I say, modern spark plugs are better at coping with the job, but do you take what someone else says as gospel, or do you like to actually check yourself..??
To some degree and I honestly don't think this would happen on our cars, there can be condensation build up sometimes within a plug, depends where it is parked for a long time. You can get some corrosion of the plug over time, but again depends on where and how the car is stored. If you found a car sat in a shed somewhere and it had been there 10 years, you bought it, would you change the plugs, oil etc, or just run it..??