This is a much more likely explanation but in all non-scientific accademic fields it's usually more important to come up with a narrative that's appealing to the general public or the political class than with an accurate narrative. Hence the explanation that makes the most sense but is less "Wow"-inducing will be swept under the rug.

There are plenty of fields where we entertain "mysteries" that have perfectly rational explanations but settling the question once and for all with a rational argument would dry up the funding stream for accademicians and could have other undesirable consequences on some countries' political narrative.

A nother comment mentions indigenous Australians.

Who, if I remember correctly, are estimated to have settled about 50kya. There are later intrusions into the peninsula, as it were back then. Oceania was settled even later by boat. The language divide in Australia is overwhelmingly that of earlier indians (if you'll excuse the pun, the land bridge was connected to the Indian subcontinent), with other language families mostly limited to the north end

This I think might strengthen the argument significantly.