The question isn’t whether DX11 would be hard to code, the question is why it wasn’t included in the first place.
Why would we even be considering a DX version lower than 10? 10 was released in 2006, how low end of a system should people have to have to use DX9? The last full version of DX9 was released in 2004, well before GW2 production was started. DX10 should have been the default, with a DX9 available as a fallback. GW2 production didn’t start until 2007, and they knew it would take years to complete. Take moores law into account, and this represents a bit of a botched job in planning ahead by ANET.
And telling people that this was already on Guru, is completely unhelpful. Not everyone trolls every other GW2 forum.
All that being said, the game is still gorgeous without DX10/11.