It is a well-known and widely repeated fact that the linguistic reconstruction associated with the comparative method is no longer effective for large temporal depths (informally estimated to be between 6000 and 1000 years, depending on the number of languages, linguistic branches and ancient documentation available). The problems are of various types:
- There is a broad semantic shift, whereby the primary meaning of lexemes is changing until the original meaning is almost unrecognizable and cognates are difficult to identify.
- Lexical replacement whereby a lexeme is substituted by another etymologically uncorrelated lexeme to mean similar things.
However, I have not seen too many studies or opinions that dare to answer more concrete questions such as:
- at what percentage of cogands shared among several languages does it become statistically impossible to find regular correspondences between etymologically related lexemes with related meanings (cognates).
- What is the weight of each specific factor, and which of all the factors weighs more when identifying cognates between very remotely correlated languages.
- Does the existence of many branches influence the possibility of reconstructing a proto-language (in European linguistics, many roots only appear in some branches and most of the reconstructed terms do NOT appear in each and every linguistic branch of the Indo-European family).