Yes, many of the old phones are not compatible, but thanks to eBay, there is a plentiful supply of only slightly older tech phones that work with your carrier at a low price.

And in fact, if carriers offered this service, they could make a nice penny doing trade-ins and reselling phones. Right now they upgrade you and leave the old phone to be put in the closet, which is a total waste.

Yes, you buy a bucket of minutes, but everybody's bucket size is based on what they observe as their usage pattern. If they find their desire for minutes goes up, they consider bumping the bucket, which the carrier loves.

Finally, this would be a competitive advantage. People would consider this in choosing carriers. Some people live with their cell always on their belt, but sometimes I forget mine, and if I could keep an old one in the car for such times, it would be highly useful, and would affect my choice of provider.