I don't think this is really that surprising, WP7 is now just past 4 years old, and WP8 has been out for two years. The average life of a mobile device is 18 months, my first windows phone (7) device died about 8 months ago, and that (I felt) was an exceptional life for a mobile device.
How many WP7 devices are still signing into skype on a regular basis? Seeing as not that many WP7 devices were sold in the first place, I doubt it's many.