I think part of English's malleability comes from there being so many 'dialects' of English. There's British English, American English, Canadian, South African, Australian....English has to be responsive to change, adapt and be accessible but that also opens it up, for want of a better word, perversion. With many languages there is ONE guardian country of the language and what they say goes. Try telling Britain it has to adopt American Enlgish as standard or vice versa! English speakers can't be as obsessive as speakers of other langauges because that would involve standardising Enlgish first and there are too many powerful English speaking countries for that to work.
However there are basic, universal, grammatical rules present in all forms of English which, suddenly, it's acceptable to ignore on the internet and that really bugs me. It means that we're going the opposite way to, for example, Spain. We're not standing up for our language, we're not protecting correct usage, we're not encouraging and correcting people to ensure the survival of 'pure' English and that to me says we don't have pride in our language. It's perfectly acceptable to speak dialect but as far as I know 'internet' is not a dialect, much as txt spk may be.
Also, it's 'normal' in France for people to correct your grammar and not just on the internet. I think it's a strange sign of affection; they want you to improve and be the best that you can be. English cultures tend to let it slide and celebrate mediocrity more.
It's a double edged sword, really!
Legacy: I like your musical analogy. Are you one of those people who is unreasonably annoyed when you don't get a decent cadence?