The discussion here is not necessarily about the DirectX version, I think in this thread about everybody agrees that LFS would have nothing to gain by moving to a newer DirectX version.
What this is about is about some people's controversial stance on upgrading software
Now, IMO If Scawen needs an XP machine to keep working on the DX9 render path, then this should have been the end of the discussion right there. Also, if your million dollar industrial/medical equipment only comes with XP drivers and the company that made it went out of business 5 years ago, that's also a pretty good reason to stick to XP for the moment (provided you also make some plans for the future).

However, to suggest that sticking to XP would be a wise course of action for the average Joe is in my opinion a mistake and it's sad to see a fellow software developer advocating this.
People here who don't develop software for a living might be excused for thinking that "all the security bugs in XP must have been ironed over the years of patching", however the sad truth is that, at the time when XP was developed, Microsoft didn't give a rat's ass about security and it's only in later operating systems that this was started to be taken seriously and features aimed at mitigating large classes of exploits have been added. This is not the forum to talk at length about Data Execution Prevention or Adress Space Layout Randomization but do look those up if you're interested.

TLDR: Sticking with XP is fine if you're a power user who knows what s/he is doing and the machine is air gapped. For the average user to stick to XP just means more powerful botnets to worry about for the rest of us.