As it is clear from the development of the dorsal reconstruction, the theory that made the fewest assumptions was that an original Proto-Indo-European had two series of velars. This should have shifted the burden of proof, already by the time when Meillet (1894) rejected the proposal of three series; but the authority of Neogrammarians and well-established works of the last century, as well as traditional conventions, probably weighted (and still weight) more than reasons. While most Indo-Europeanists would find the large inventory of consonants in the reconstructed Proto-Nostratic as methodologically primitive compared to most Indo-European sound laws (Kallio and Koivulehto 2018), tritectalism—just like the need to reconstruct laryngeals for every vocalic difference between dialect—does not follow the same methodological standard for Indo-European studies.
More than half century ago we had already a similar opinion on the most reasonable reconstruction, that still today is not followed, as American Sanskritist Burrow (1955) shows: “The difficulty that arises from postulating a third series in the parent language, is that no more than two series (…) are found in any of the existing languages. In view of this it is exceedingly doubtful whether three distinct series existed in Indo-European. The assumption of the third series has been a convenience for the theoreticians, but it is unlikely to correspond to historical fact. Furthermore, on examination, this assumption does not turn out to be as convenient as would be wished. While it accounts in a way for correspondences like the above which otherwise would appear irregular, it still leaves over a considerable number of forms in the satem-languages which do not fit into the framework (…). Examples of this kind are particularly common in the Balto-Slavonic languages (…). Clearly a theory which leaves almost as many irregularities as it clears away is not very soundly established, and since these cases have to be explained as examples of dialect mixture in early Indo-European, it would appear simplest to apply the same theory to the rest. The case for this is particularly strong when we remember that when false etymologies are removed, when allowance is made for suffix alternation, and when the possibility of loss of labialisation in the vicinity of the vowel u is considered (e.g. kravíṣ-, ugrá-), not many examples remain for the foundation of the theory.”