There are 3 components to any language: the sound system, the grammatical patterns, and the stock of words. For a language to exist and persist, these may be allowed to vary, but within limits. As soon as the variations start to interfere with meaning, we can say the language has deteriorated. This will be an objective statement, because it is now less fit for purpose than it used to be.
The components of the language should be taught to everyone using it: you can't simply wish that everyone starting in first grade knows all he needs to know about it. So you need to teach the sound system, the grammar, and the vocabulary.
Now, taking American English, we find a marked deterioration in the sound system over the last two decades or so. This came about because of the influence of CNN, at first, where they decided deliberately to use a Midwest accent to be comprehensible, as they thought, to the majority of Americans. The result is a marked decline particularly in the vowel sounds of AE.
Also in AE, as has been noted, schools no longer teach grammar, so students are left to make it up as they go along, with predictably awful results. And imagine trying to teach a child who has absolutely no concept of grammar a foreign language!
Vocabulary has suffered too. While we have many more nouns for concrete things - because we have many more concrete things - we have added only a handful of verbs, while dropping almost the whole class of adverbs and retaining only two adjectives - cool and gross. Or so it seems to a detached observer! And we have lost many abstract nouns, in parallel with the loss of the power of abstraction.
If you think this is a harsh evaluation, just get yourself an old Hitchcock movie - without subtitles - and compare it with a modern piece of trash from Hollywood. You would be forgiven for thinking that Americans of 50 years ago did not speak AE.
Language is much more than a tool of communication, and it communicates much more than can be put into pictures. It informs, controls, and directs thought. It is the one art form instantly available to everyone. It should be something we nurture and cherish and delight in, instead of something we want to reduce to the simplest sequence of grunts necessary to convey our desire to get an iPad and log on to Facebook.
I think it can be demonstrated that the utter banality of American public discourse, where the clowns running for election would have been laughed off the platform a generation or two ago, is the result of the dumbing down of the American people and the American language. The only solution is to set up an academy to preserve (without pickling) the language, and appoint evangelists to go into all the schools to teach people how to speak, and therefore how to think, and how to conduct themselves in a civilized world. Perhaps there is still time!
English developed from Latin, Greek, French, German etc and got rid of 
