You seem to have a strange definition of "opinion" ...
> What is opinion is that some people think
> that there are hard rules to language ...
No, it is a fact that some people think that there are hard rules to language.
But all this is beside the point. Yes, you are absolutely right that language is fluid, changing, different in different places, and trying to produce "proper rules" and "proper grammars" (by some hard-to-define meaning of the word "proper") is like nailing jello to a wall. You can try, but you won't have much luck.
That being said, there are some "rules" which, if you follow them, help you to communicate effectively most of the time. There will be exceptions, and there will be geographical variances, and these are not rules of the language, but rather, they are rules for you to follow while synthesizing constructions.
To be honest, I expect you are in violent agreement, and arguing with attitudes you have found elsewhere, but which I do not hold.
It is a fact that some people have the opinion that there are hard rules to language. Better?
More seriously, I think we're in violent agreement, the difficulty is how we parse the word "rule".
If rule means "something that must be done or you'll be a bad person and the world will end" as many language prescriptivists like to use, then it is a fact that languages do not posses rules of this sort.
If rule means "a mutual set of understood guidelines" then yes, I'm in full agreement that languages can have sets of customary rules that aid construction and understanding, but this set of guidelines is incredibly fluid and can change as you've described including temporal, generational, socioeconomic, ethnic, fluency and other sources of variation that can alter the set so profoundly that two speakers of the same language can be more or less unintelligible to each other yet still be said to be speaking the same language!
Back on topic: "a green great dragon" sounds awkward unless we're having a discussion about the colors of "great dragons" in which case it's perfectly ordered and "a great green dragon" makes no sense since we're not talking about sizes of "green dragons".
But all this is beside the point. Yes, you are absolutely right that language is fluid, changing, different in different places, and trying to produce "proper rules" and "proper grammars" (by some hard-to-define meaning of the word "proper") is like nailing jello to a wall. You can try, but you won't have much luck.
That being said, there are some "rules" which, if you follow them, help you to communicate effectively most of the time. There will be exceptions, and there will be geographical variances, and these are not rules of the language, but rather, they are rules for you to follow while synthesizing constructions.
To be honest, I expect you are in violent agreement, and arguing with attitudes you have found elsewhere, but which I do not hold.