The classic theme supports arbitrary sizes (in px or "device-independent" units) for both text and widgets, which addresses HiDPI and touch readiness. What it might have trouble with however is mixed DPI, where moving a window across two screens requires rendering it with different pixel densities. But that was not on the horizon in the Windows 8.x days.
It doesn't always produce the most beautiful results, but it's serviceable. And to be fair, it's taken a long time for high-DPI to consistently look beautiful even on recent versions of Windows. I shudder to remember looking at Device Manager on a display set to 125% in the DPI settings.
This claim is entirely baseless. The "classic theme" is just a gray background plus some light/dark borders. There's absolutely zero difficulty doing that in hi-DPI, or any other environment for that matter.
Stop making excuses for Microsoft. Windows XP was a panicked and half baked response to the shinny Mac OS X Aqua interface, because that's what Microsoft had always done: following Apple's lead. I can't really blame them either, as following Apple's lead had led to great success for Microsoft.
Actually I lied, I do blame Microsoft for blindly chasing Apple's new shinny. Having established themselves as a pioneer and authority in UI design/usability with the revolutionary Windows 95 GUI, they should've had the confidence to stick to their guns and iterate their GUI without too much influence from the competition.
You could in theory design a modern UI that resembles the classic UI, but it wouldn't really be "the classic UI". A large part of what makes the classic UI have the particular look it does is that it has custom constraint-layout logic for each control, the outputs of which are fed as parameters into the arbitrary hand-written GDI draw-call logic used to draw each control.
Some of the more interesting controls have Turing-hard constraint logic, which just can't be replicated in non-Turing-complete constraint-layout systems like XAML or CSS. (You can get close, but it requires using huge hierarchies of layout nodes with tons of overhead, where the Windows UI actually got by with very few.)