lots of compilers have been written by a single person, including compilers for c++, d, turbo pascal, forth, scheme, ml, haskell, ancient lisp, and so on. craig burley got g77 to the point of compiling useful fortran programs before other people got interested. i think graydon wrote the first versions of the rust compiler himself too
it's probably always better to do it with someone else, because you get better ideas and faster debugging, but pretty much all hlls are possible for a single person to implement