I mean not explicitly hardcoding the path to the interpreter inside every binary. If the dynamic linker is part of the OS, then surely the OS should already know where it is?
It's both part of the OS and the language's compiler tool chain the program was written in. For the vast majority of Linux executables that's glibc and the ld-linux.so that glibc ships.
And you don't need to hard code the path inside your executable. Static PIE objects for example are dynamic elf objects without an interpreter header. The system loader is capable of launching them just fine.
I ask because ELF and unix dynamic linking is a pretty well thought out set of specifications and processes and it's a huge body of work. It can certainly be criticized, but calling it an afterthought because of .interp doesn't seem very charitable, just trying to understand if that's an informed opinion and if so it would be interesting to hear more.