[prerok]

Can you cite sources where this is argued?

The wikipedia seems to make a clear definition:

https://en.wikipedia.org/wiki/Language_death

That said, the above article does use extinction and death somewhat interchangably later on, but I suppose it's almost the same for small languages that nobody learns who is not a native speaker.

[nephihaha]

There are plenty of languages which exist without much in the way of L1 speakers. Esperanto, for example, although it does have a handful of native speakers. Many people speak English as a non-native language, particularly in places like India or Nigeria. Swahili was originally a trade language few folk spoke and even today, many of its speakers are L2.

I can speak and read some Manx. I personally don't believe it died in the 1970s. Not only do we have continuity from that time, there are people around today who learnt theirs off native speakers (in one case they were his close relatives.) It helps that we have many recordings, writings etc and it is also closely related to two languages which are in slightly better shape.

Latin and Hebrew were in use within the Middle Ages to a substantial level and used to communicate between people as a common language sometimes. Hebrew is now revived, but is Latin? A few people have spoken it as their first language over the last century or two.