The Correctness of English: A Decaying Language

I often hear conversations and ask, how do they understand what each other is saying? However, I think nothing more than that and certainly voice no opinion, during my research into the English Language, I stumbled across a particularly interesting group of people, perscriptivists. They share a belief that English is a vulnerable feature of our lives which must be protected from the decay threate would be hard to deny that I, as anyone calling themselves a writer should, believe in a certain standard of language use, both written and spoken, though to myself written quality of English holds more importance. However, if one were to read the introduction to the English Dictionary, written by Samuel Johnson in 1755, he says:

"We laugh at the elixir that promises to prolong life to a thousand years; and with equal justice may the lexicographer be decided, who being able to produce no example of a nation that has preserved their words and phrases from mutability, shall imagine that his dictionary can embalm his language, and secure it from corruption, and decay, that it is in his power to change sublunary nature or clear the world at one from folly, vanity, and affection."

Dr Samuel Johnson, described by many as a perscriptivist of the English language was under no illusion that language could be "purified" and "preserved", he knew language would change, it has always been inevitable and has, since its roots, changed.

So why are there then some people who believe language is decaying and should be protected, since within history, there is huge sign of change? The English language as we know it is made up of multiple European languages and changes between each country which speaks it. Then is there really a case for a decaying language?

I understand the view that language should be of a high quality, and all who need should have access to levels of education where English is taught at the same standard, but how can we remain equal if each persons view on language, and what they expect of it, is different? If a variation in lexis happens from region to region, let alone throughout the world, can we say one form of this language is more equal than another? Taking Spanish, spoken by the Spanish and with historic invasions, is now used throughout much of south/Latin America. With three different versions between the home country of the language, Mexico, and the South American Continent (Excluding of course Brazil), is one form of the Spanish Language classed as more equal or correct than another? Or is it simply different?

Of course English is not the only language under such scrutiny, I just wonder why when we live in what is called an equal, free society, we should be judged on the quality of our use of language alone. It seems as though the power which language gives us is still in this day and age, limited to the select few who are fortunate enough to have what is considered the right language use along with the right accent.