As brought up in earlier replies, the main problem remains that the current Python build system is a hybrid that has evolved over the past decades. It is also two separate build systems, one for Windows and one for all other (Unix-y) platforms with various platform-specific tweaks. The various competing build systems, like Autoconf, Autotools, and CMake have evolved greatly over the years, for example in support of cross-compilation something that we have to painfully hack on in the current build system every time someone proposes supporting a platform via cross-compilation.
A big complication is the Unix-y build system’s bootstrap use of the interpreter being built, and its copy of Distutils, to build much of the rest of the standard library. Because of the many ways Python’s Unix-y build system has evolved its own solutions to various issues over the years, it doesn’t necessarily automatically take advantage of new features provided by new releases of Autoconf and trying to hack them into the current state of things is likely more painful than starting from scratch. There is also still cruft in the current build system that was a result of support back in the day for platforms and releases that we no longer support.
Especially now with the current efforts to deprecate and eventually move Distutils out of the standard library into setuptools while possibly leaving a version of Distutils behind as a build tool just to build the standard library itself, it makes sense to start with a bit of a blank page and try implementing potential build system replacements using one or more build systems using each’s recommended best practices and see which seems to work out best. That would give us a much more solid base to build on for the future. It might also be an opportunity to move to more of one build system, rather than the two we have today. although that isn’t necessarily an over-arching requirement as the current Windows build system is platform-focused and thus likely has much less residual cruft. But that’s all going to take some focused effort probably best done as an official, funded project. Doing anything less at this point would be mainly wasted effort, IMO.