It's sometimes impossible to detect sarcasm from just text, that's why Poe's law exists. You may be good at understanding sarcasm and satire, but some people aren't and putting /s is making sure that everyone understands instead of just you.

I feel you on the dumbing down part though, but I think sarcastic comments are not a form of media that must be left only to be enjoyed by the people who are "better than this".