Since the natural descendants of the PC are the XT then the AT, I believe that our PCs are in fact ATs, is that correct ?
No. If at all, they are PC-Standard Computers, as this group, lead by Intel andMicrosoft was the only standard body trying to define and standardize the advancements.
In terms of compatibility thay are no longer defined by some hardware API (*1), but software API's - thanks to an ever increasing CPU power enabeling the encapsulation of hardware to a great extend. The last of these common standards is the PC-2001 from, well, 2001. One reason for not adding any newer is that the 2001 standard did not only cover next to all common user related interfaces (and removal of obsolete), but also is seen as the point where the Legacy-Free-PC came to be.
It was based on the fact, that individual interfaces where no longer developed. Standards like PCIe and USB covering all I/O connections with new deviced only added thru one of these. All together hiden by multiple levels of API - just this time way too complex for average programmers to hack around :)
*1 -in fact, the PC was never realy to be defined by hardware, it was just badly written software bypassing abstraction layers that resulted in hardware dependance.