I don't agree.

Every language evolutes, there was a language which Latin came from. And Fusha must come from another language. Indeed I am Spanish and we don't speak exactly the same language you can read in book from 300 years ago. Actually these days nobody speaks exactly as in the "Quixote".

Actually I have to disagree a bit.  This isn't exactly how either Latin or Fusha progressed.  There is debate as to whether Fusha itself was ever anyone's native language and, whether or not it was, it was itself a dialect or combination of dialects.  Classical Latin was a standardized form of Old Latin (which would of course been a dialect) that was spoken by the educated classes (much like Fusha may have been in the Middle East).  Anyway, once both Latin and Fusha left their birthplaces they quickly became divergent and evolved, because that's what languages do. People did start writing their version of Vulgar Latin and now we have the Romance languages. If more people start writing their versions of colloquial Arabic they'll become their own languages (truthfully many of them are already, just not commonly written yet). This doesn't mean MSA or Classical Arabic will die, it would just shift positions in society. We can have Shami and Masri and Darija and Khaleeji and still have MSA and Classical Arabic.  Also, note that none of these is "slang".  Slang only means the most informal register of a language.  An Arabic عامية isn't really what an English speaker would call slang because it serves daily (and even some upper level) functions slang can't.  A clearer term might be colloquial.  

Nice post bro, thank you :)  One point that I must disagree on is that they didn't have mass media in the days that Latin developed into other languages.  With TV and the Internet, I'm sure that Fusha will survive.

You're right and I appreciate your suggestion.