I would simply say that if an app accesses undocumented low level routines then that app can equally expect to break as soon as fixes or a new releases of the OS emerge.
I doubt that was the problem with XP apps not running under win7. Windoze apps stopped accessing hardware directly a decade ago or more.
From what I have seen, the issues are (e.g.) the long standing but dodgy practice of storing data/config files under the Program Files directory, and other places. Win 7's improved security stops stuff like that.
it doesn't use memory as HDD cache
XP does cache heavily. Try starting a say 100MB app twice. The 2nd time is much faster. NT4 onwards.
it wasn't designed to use more than 2 CPUs
Plenty of apps do use 4 cores under XP, but it has to be coded in the app, obviously. Despite massive research over decades, nobody has yet found a way to efficiently use multiple processors to run a single-threaded app. Maybe the OS itself doesn't but that hardly matters.
not if you want to fully use today's hardware.
Which "today's hardware" requires win7? Maybe something obscure comes only with win7 drivers. When I bought the win7 PC for my GF, I asked for XP and the firm said a blue ray drive won't work under XP, so reluctantly I went for the win7 option. This proved to be a total con, since I am happily writing BD DVDs on every XP machine I have, even using a USB writer for $150.
Uisng > 3GB RAM is nice; I agree, for high end apps like video editing. But Sony Vegas runs just fine, for editing my absolutely gripping 2 hour VMC on top flight movies
The problem is that any "old" PC user ends up finding so much stuff stops working under 7 so a lot of stuff one has paid for, and which works, has to be chucked out, and most of it is unsupported so you don't get updates, you lose the familiar functionality and have to splash out more £££ to replace it. OK in a corporate scenario, sure.
XP is buggy and an XP machine rarely makes a month without a reboot, especially if you use bodgy apps like PC/Anywhere, but this is a complete non-issue relative to the productivity and the ability to run a huge range of "legacy" (don't I hate that word) software.