>>86
BIOS development tools are still almost exclusively DOS-based. There's no need to change what works. Updating the BIOS, various other devices' firmware, and basic hardware testing and recovery are some other uses where DOS is still very much alive.
Powering on in AT compatible mode just means that every modern OS has to jump through extra hoops to get the system into a state that's actually useful.
Those "extra hoops" are, once again, minimal. The PC architecture (and x86) has lasted so long precisely because they've followed this very important principle of backwards compatibility and progressive enhancement. No one wants to buy something that won't work with what they already have, and telling them to just upgrade everything is arrogant and a ridiculous waste of time and other resources. No one wants to relearn everything about the hardware and software, nor rewrite all the documentation. One only has to look at the failure that was Itanium to see this in practice.
Contrast this with the Raspberry Pi: no one except Broadcom really knows how it works, any other SoC is likely to be very different and require completely new software, and I'll be eating my hat if they still make the chip or a compatible version in 10 years. It was designed for planned obsolescence, an evolutionary dead-end.