Absolutely normal. What percentage of the original sold C64s were still in use 8 years after? What percentage of the original IBM XT clones were in use 8 years after they were produced? What percentage of the 603 and 604 based Macs didn't disappear or die since 1998?
I think this is a different situation. First, the PPC cards still have a substantial cash value attached to them. And so, when a person ceases to use them, they tend not to sit unused but to be sold on to another user who, having invested a relatively large amount of money, is going to use it. A C64 / old Mac / XT/AT PC, on the other hand has no value, and so is more likely to be left in a closet gathering dust.
Secondly, in the case of the 603/604 Macs, they have been superceded by newer / better / faster models. While in the case of the PowerUp cards, they are stil the fastest expansion available for the classic Amigas. Again, this encourages people who actually own them, to use them.