Most oldest (archaic) indo-european languages.

In fact until recently Italian was not spoken by all the Italians. It was since the appearing of mass media when Italian spreaded to all the country. Before the unification of Italy, Italian was the language of classic authors like Dante, but the languages spoken by the italians diverged greatly from what is known as Italian, even the dialect of Tuscany. So it could be considered as a dead language used only when writing, but not by native speakers. After the unification of Italy it was slowly reintroduced.

It couldn't have survived if there were no native speakers. You're right ,at first it was just another simple dialect,which then spread due to the need of a common language. The same thing happened with Spanish,French and Romanian.But I just don't see how it could have been dead at any time. If it was used in official writing and books must have been because a large amount of people understood it and thus it's very unlikely that it didn't have any native speakers.

No, it was not a dialect, it was a fossilized language not used by native speakers. If in Northern Italy they learnt Italian and nowadays it is the native language of most of the people, why are some native speakers necesary in order a language to survive? Even if the Italians chose Latin it would be nowadays their native tongue after generations of people who studied it at the school and were encouraged to use it all the time. Languages are similar to living beings, but they are not the same. Languages can resurrect.

That's true. Hebrew, for example, had no native speakers for a long time, but now it has many.