How expensive would it be to manufacture a feature phone that is compatible with 2G, 3G and 4G LTE with VoLTE support? I think there is a good market for such a product.
Try buying an international phone with the correct band support and put it onto a US network, it'll kick down to 3G or 2G to make the call. No point really in having the extra data speeds a featurephone won't need (for most people). I personally would like to have a small, durable, and low powered featurephone I could also use as a usb modem.