There is a new optional update from Microsoft. If you choose to take this update, your phone will be upgraded to a non-supported build of Windows Phone 8.0 for legacy devices. If you choose to decline this update, you will be able to receive fully-supported updates of Windows Phone 7.8 and beyond. This update will wipe all your data and could potentially brick your device. Expect poorer performance. Microsoft and its partners are not responsible for any damages caused by this update.
Would any of you be objected to the above statement appearing the next time you connect your device? Do you think your arguments against WP8 on our devices stand up, taking this possibility into consideration. Yet again, it boils down to this: no one seems to understand that giving choice is actually an option.
I'm 99% convinced that the biggest technical problem (i.e. avoiding things like "not worth the effort") is that manufacturers wouldn't bother to write drivers for these old devices on the new kernel.
Performance is not a problem - Nokia needs to bring Windows Phone to even lower price points, and that won't be possible with kick-*** processors. The Lumia 900 processor should absolutely be able to run WP8. Joe Belfiore himself said that the NT kernel allows then to scale both up and down.