Speaking as a survivor of the 80's (you thought we were all gone, didn't you), "everyone" didn't write in assembly language, only some of us. We really did have other languages then - C, Pascal, Fortran, Lisp, Cobol, Basic, tons of others. No Python or Perl quite yet, but lots of Awk and Sed and Yacc and Lex and shell scripting.Just remember what Bill Gates is famous for saying... "640K ought to be enough for anybody". So, never put a ceiling on memory use or need. The future is ALWAYS calling.
However, I'm sure if everyone coded in Assembly Language (like back in the 80's)... RAM requirements and processor requirements would be a lot lower. I mean, realistically, how much space RAM/disk SHOULD macOS Monterey take up, if coded in 100% pure Assembly Language? What generation of processor could macOS Monterey run on, if coded that way?
Writing in assembly can get you some space savings over C, but not as much as you think - C is basically a generic high-level assembler (and it's still the language that most operating systems are written in). What makes something like Monterey big is not the choice of C over assembly, but rather... just how damn complicated it is. It's doing so many things. Oh, and if we were writing OS's in assembler these days, we'd be at least a decade behind where we are now in capabilities of the machine. The reason for writing in C rather than assembler, is that little extra bit of abstraction makes it much easier to reason about and write large systems.