We're not talking about artifacts though; we're talking about people.
Yes it would be better to be able to observe and use a language within a native, living body of people in order to be able to well comprehend and thoroughly appreciate the nuance, breadth and depth of a language. But in order to do that you also must require that population not to use some other language which may well benefit them (in terms of education, trade, international relations, access to information, etc.).
In Wales for example the historic Welsh language is being propped up at vast expense to a relatively poor region of the UK. All lessons¹ in [state run] primary schools (4-11 year olds) are used to promote the historic Welsh language and children are taught that their own language - predominantly English - is to be deprecated in preference to Welsh.
None of that serves the children. Without government interference, if trends continued, barely a handful of people of any age would speak historic Welsh. As it is now - made compulsory for all children in state schools - use of historic Welsh language is still declining.
That would indeed be a loss to those interested in Brythonic languages or who mourn for the simpler times of their youth when they spoke historic Welsh or who despise the English people so fervently that using "their" language is anathema [I'm not even joking], or whatever. But, would it be any loss for the people living in Wales as a whole? I warrant not. Indeed it seems it would be a gain.
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¹ PISA results for 2012, http://priceonomics.com/only-4-of-languages-are-used-online/, are just in and show Wales is falling behind the rest of the UK and that the UK is slipping relative to the other countries surveyed. I think an insidious over-emphasis on historic Welsh is at least in part to blame.