Modern linguists believe it's natural that languge changes. Similarly, scientists believe it's natural that if your are not careful with your diet, you get fat.
If we just relax our rules on how we write or speak, the communication between our generation and the future one will break. Shapespeare is slimly understood. Chaucer is completely off throwing. Sadly to say, these are two of the first and greatest English poets.
If we are careful with our speech and writing, and if we follow the rules of good grammar and English, our language of today, should still be understood 400 years later.
Untill we speak and teach our kids the language that is documented and accepted in Standard English, our literary heritage of today will be unfathomable, unrythmical, and at best boring and not beautiful to the future generations. Don't you think there is a good reason that we strive to be purists?