What use do we have for a build system that is less capable of the one that is very widely used, is available on nearly every platform, and on the ones where you're likely to need it most, it's usually already installed?

The biggest falsehood is that programmers want such a build system. The chance of this system replacing Make is zero. The only way a build system is ever going to replace Make is by being 95% backward compatible with Make. If you're intentionally adding limitations to your build system such that most existing Makefiles can't be directly ported onto it, then your build system will never gain wide use beyond your niche cases.