So you would rather have them waste that exact same amount of space on every computer, just so you don’t have to download it when you want to build a package?
Header files are literally less than a couple kB per package, and Megabyte-sized headers are the exception for huge frameworks like QT or GACL. Arch packages don't ship the source codes, only the binaries (.so files) and headers (.h files).
Overall, the /usr/include folder on my system has less than 230MB, which is a damn lot because I have all kinds of compilers, programming languages and libraries installed. I even have 32Bit and atmel libraries installed because I do embedded reverse engineering and cross-compiling development on other architectures; so I also have gcc, LLVM, rust, go, C/C++ and other programming languages and their dependencies installed. So it's very likely much less than that for others.
If you cannot afford that, you should maybe choose another, smaller, distribution like "Damn Small Linux", Alpine or similar. Debian wouldn't be a good choice for that matter anyways.
Neither do Debian's -dev packages.
Multi-stage builds do make it a bit easier to get rid of them for containers, yes, but during the build stage it's still required.
Absolutely, it's negligible - because the space consumption of header files is also negligible. It's such a bizarre design choice to split out a single 30 line .h file into a separate package.