I would think it is not beyond the wit of man to be able to retrofit MCU3. If input voltage is the only practical issue, the boards can be reworked, or other stuff added in the electrical pathway to step up the voltage.
If there is sufficient interest I'd assume Tesla would investigate it. But Tesla being Tesla there is every chance they would simply say that people need to upgrade their whole car to get it, which is a departure from previous attitudes (i.e. MCU1 cars can upgrade to MCU2, etc).
I've some sympathy for that argument and indeed the MCU 1 and 2 differences were quite big with respect to the AP hardware. On the one hand they managed to make it work with AP1, and on the other they seemed to give up with HW2 and HW2.5 and it was easier all round to standardise on HW3, this may be software as much as hardware related, but still shows the complexity of these things starts to mount.
I know I'm beating a dead horse, but the promise when we paid for the FSD option was that all hardware necessary to achieve FSD was included. Whilst it is a long way off, when (if) it ever makes it to the UK I will expect that they live up to that promise. I am not holding my breath ....
I think others have commented, AP hardware and MCU are very different. I know some got upset when things like sentry mode didn't work on HW2 cars and wanted free upgrades which Tesla refused. Radar is still being shipped on MS and MX as well as China M3 and MY (unless things have changed very recently), the interior camera isn't on all cars, and I can see Tesla being creative what they try and pass off as FSD and what isn't (for instance holding the steering wheel is still needed without the camera, or passive safety is dependant on the radar but thats not classed as part of AP. It was a bit of a bold promise to say the cars would have all they needed and at some point I suspect will bite Tesla. #popcorn-at-the-ready.