Re: It wasn't just apps
>> Windows Phone 7.x devices couldn't run Windows Phone 8.0. WTF?
> IIRC, the argument was Secure Boot or something.
WP7 devices were strictly single core SoCs. WP7 was based on 'CE' and couldn't cope with more than one. In fact the advertising had 'why would you need dual core' implying that others were inefficient.
WP8 _required_ dual core and all the SoCs that it would run on were specific dual core parts.
Microsoft only built versions of WP7 and WP8 to work with a specific list of SoCs and there was no overlap. The makers could not rebuild for a SoC not on the list. In fact originally each maker was directed to use a specific source.
> A certain HTC HD2 disagrees. Not only it ran WP7, it also ran WP8, Windows 10 Mobile, Windows RT, and Android
It seems that the HD2 (a Windows Mobile 6.x phone) was used by MS to develop WP7 as both WM6.x and WP7 were based on CE. This allowed developers (not HTC) to hack WP7 to run but is was unsupported.
"""In December 2012, Windows RT and Windows Phone 8 were ported onto the device as a proof-of-concept; no working builds exist."""
While the WM10 upgrade list does have an "HD 2" it is not the 'HTC HD2', it is the 'Blu Win HD 2'.
Android (ASOP) is, of course, available to be built for any device.