People here can say that asking Microsoft to supporting XP is ridiculous.
But is it really so odd to do what your developer customer base wants?
Head on over to Visual Studio User Voice and look at the top requests. You will see that XP support is number 5. And all the ones above it are C++ and IDE related.
So the top .net related request of users is to add XP support for .net 4.5.
This is all because developers have lots of customers that are using XP. We can't make them change. But we can't make Microsoft change either.
So will all try to cope with the mess of the in-place upgrade as best as possible (my corporation has blocked .net 4.5 from all computers).
But in the end it would have made everything so much better if XP support was there. Or failing that, make .net 4.5 a side-by-side install so that it does not mess up development of .net 4.0 apps.