The following excerpt has been posted at Gaidhlig-B, so I expect we will see further discussion at that site.

"From The Guardian Weekly:

'Ancient Tongue Falling Silent

'The [2001] census has also brought gloomy news for the dwindling band
of Scottish Gaelic speakers, now down to fewer than 60,000. That is
perilously close to 50,000, the level at which the ancient tongue would be officially considered dead, and a 15,000 drop on the 1991 figure. The language is more geographically restricted than ever before, with a high concentration in the Hebrides, and only a tiny minority in the central region of Strathclyde. Nationalists in the Scottish parliament want legislation to give Gaelic equal status with English, but that is being resisted by the Labour-dominated executive.' "

Gaidhlig-B, bi-lingual discussion about the Gaelic language and related matters; and Gaidhlig-A discussion, about anything, only in Gaelic, can be accessed via http:/www.smo.uhi.ac.uk/liosta
(as you might expect with a virtual community, a lot of the correspondents are Gaelic speakers and learners who don't reside in Scotland)

obviously this short report in the Guardian Weekly doesn't fully explain the issues(politics of secure status for Gaelic, etc), and it doesn't answer my query about why 50,00 = "officially considered dead"