Devices with no clue of IPv6 also just drop those packets. What have we gained?
The only thing I can imagine is that while you still need to alter or replace every piece of equipment on the net, software adoption would likely have been much easier and thus immediately higher if 128 bit addresses were the only change (I still don't see the benefit of tucking it in as a field into IPv4, but if IPv6 was just IPv4 with wider addresses), and all the other protocols and semantics that were changed with IPv6 stayed the same. But arguably, since you do need to change every piece of equipment, this was the time to make desirable, fundamental, not-backward compatible changes, and possibly the only opportunity at that.
The only thing I can imagine is that while you still need to alter or replace every piece of equipment on the net, software adoption would likely have been much easier and thus immediately higher if 128 bit addresses were the only change (I still don't see the benefit of tucking it in as a field into IPv4, but if IPv6 was just IPv4 with wider addresses), and all the other protocols and semantics that were changed with IPv6 stayed the same. But arguably, since you do need to change every piece of equipment, this was the time to make desirable, fundamental, not-backward compatible changes, and possibly the only opportunity at that.