Too many people use "language is fluid" or "I only care about the message" as excuses for being lazy.
Your post is a display of ignorance. I'll dismiss your point about lazy because I never said anything about it and have no intention of bothering with it.
The issue about language is that it is not stagnant. If it were, we'd still be using grunts and squeals to communicate. Language is continuously changing. It moves, it is alive. Dictionaries add new words every year. This is something that everyone knows, every educated person. I have a master's degree in English. This is a concept I learned in undergraduate school. Our language is constantly changing, i.e., it is fluid, not stagnant.
I have letters my father wrote to his mother during WWII. The way he used language in the 1940s is not the same way he used language in the 1990s because the way we speak, the way we write, the way we communicate changes. This has nothing to do with people being lazy; it has to do with people communicating in the accepted way for their time.
I agree that saying I should of rather than I should have is a basic mistake. People make these mistakes all the time. They use verbs incorrectly, they use vocabulary incorrectly, they do not punctuate their sentences correctly, and so on. If something is done often enough and is accepted as general use, it is incorporated into our language. For example, few people know when and how to use either who or whom. No one seems to care and no one on this board ever corrects those who use the incorrect pronoun. So, it has gone out of general use to differentiate. Even English teachers often don't know. Is this a big deal? Do you know, without looking it up, when to use who and when to use whom? In fact, the word whom is rarely used anymore; it has become nearly archaic. Another thing is the dangling preposition. People use them all the time. It is commonplace. No one corrects them, or rarely, and an awful lot of people don't know what it is. It is another thing that is falling by the wayside. Is this terrible or is it just the evolution of our language?
Every culture, uses its language in the way that suits the life of the people. Language reflects the people who use it. Our English is not the same as the English in the UK, Ireland, Australia, New Zealand, or Australia.
Language is a living thing.