What happened to the Gallic language?

This has been racking my brain for a while now; EB has taught me they were literate, or at least, had a written language for use by nobility. Even the Galatians had this ability judging by the inscription in Egypt.

EB used research that had to a degree reconstructed Gallic, so my question is; What the hell happened to Gallic? there's not another case in history I can think of where an entire major language/culture could be totally wiped out without almost no influence on the replacing language (except phonetic elements of French i'd assume), nor have some surviving form today, unless you count the example of South America, but that was more or less a full on genocide. So does that mean Caesar did not exaggerate in his De Bello Gallici with his near genocide, or something?

I don't wonder about this for other lands; Hispania lacked an overarching culture/language, for instance, so Latin's hold and ability to kill many smaller languages isn't a surprise. But despite political differences, Gaul seemed to have an overarching culture/language to at the minimum infuse with, rather than acquiesce to, Latin. I've read of small Gallic communities surviving into the late Empire, but probably as their modern Minx equivalents.

So, what happened? you guys are the experts, or at least, know the experts. :)

Writing was forbidden due to religious laws and cultural traditions...
Also writing would've reduced the druidic class importance...
But as you said few communities survived (in the British Isles), with christianity the upper classes now adopted writing and possibly became bilingual with latin...
In the mainland they all adopted latin or greek, even though, in some cases (Galatian, Galicia) the local tongue survived well into the 1st and 2nd century AD...

Today Breton, Cornish, Welsh, Gaelic, Irish and Manx are the descendants of the Brythonic and Goidelic languages akin to what Romanian, Spanish, Italian and French are to Latin for example...

You have to remember that Gaul was under Roman rule for 500 years. Within that time Latin became the language of law, power and commerce, if you were a progressively minded Gallic leader or merchant then Latin was the language which opened up career prospects. Gallic, by contrast, developed the stigma of being the peasants language, much as Scottish Gaelic did following the Norman influence in Scotland. Gallic didn't completely die out, a few French words today (mostly words associated with agriculture) are of Gallic origin and Gallic played a major role in modern Breton.

In a nutshell languages tend to begin to become extinct once they cease to be tongue of trade and/or power unless they are maintained by a strong sense of nationalism and dedicated speakers (the reasons behind the survival of Welsh.and Finnish despite years of English, Swedish and Russian control, respectively).