I agree that Skymiles don't expire but they do devalue over time. From my view, Skymiles have lost value at ~5% annual rate over the last 10 years when looking at domestic economy tickets and at ~15% annually over the last 10 years when booking international business class tickets.

I'm sure there will be many that come back and defend / dispute my premise that Skymiles continue to devalue over time and how all you need to do is be flexible and you can always find the ~100k business class ticket to Europe. I'll add that it's great for those that see their Skymiles as having a stable or growing value over time. From my end, I use them as I earn them when I am happy on the value I get from them versus purchasing such tickets in cash.