It doesn't need to, and reluctance is not wrt. to the proposed new layout, but wrt. changing the current one. Tons of infrastructure depends on the files having exactly the names that they have now, and being located in exactly the locations where they are currently located. Any change to that, whatever minor, will cause problems to some people.
Just to be absolutely clear: You are talking about the build environment, right? Because I am not proposing to change any layout of the installed Python (wherever that may be :)
Correct.
I am baffled about why the build environment's layout matters, but once an .msi install can place the binaries in any old place it wants. The build structure doesn't have to reflect the final installed structure at all.
No. But still, people like to be able to "run" Python out of a source check-out. This has been supported for a long time, and more and more stuff was added to support it. For examples within Python itself, see the support in distutils, getpathp.c, PCbuild/rt.bat, and Tools/buildbot/*.bat. Reportedly (by Mark), building Mozilla (the web browser) also "knows" about PCbuild. Regards, Martin