From what I understand, the general consensus of Historians until recently was that the modern Japanese people are a product of the heterogenous Jomon peoples (arrived ~15 000 BC, comprise ~10% of modern genetics) and the Yayoi people (arrived ~900 BC, comprise ~90%). The Japonic language family is attributed to the Yayoi people, who brought their language from the Korean peninsula (Peninsular Japonic was later replaced by the Koreanic languages), and the language spread across Japan with the only recorded remnant of Jomon language being the Ainu language.

However, the new Tripartite theory of 2021 instead suggests that Jomon and Yayoi ancestry account for only 13% and 16%, respectively, and that the remaining overwhelming 71% is from migrations during the Kofun period (~300 AD). These 'Kofun people' appear to be related to the Han Chinese, and the period corresponds with an influx of Chinese artifacts and language.

Under this model, does this mean that the Japonic languages are still associated with the Yayoi, but the Kofun period resulted in the massive influx of Chinese loanwords into Japanese, forming a Japonic substrate and Chinese superstrate that we see today? If so, why would the modern Japanese language remain fundamentally Japonic in core vocab/grammar, rather than being replaced by a Chinese Language with Japonic loanwords, given that these Kofun-Han people made up the large majority of the population.

For comparison, in the Norman conquest of England, a population of a small Old-French speaking elite resulted in a still Germanic language but with ~50% French vocabulary. The Kofun migration would seem to be much more similar to say the Anglo-Saxon settlement, where the native Celtic languages were completely replaced by Germanic.

Perhaps this question would be best for r/linguistics, but I'd really appreciate conversation about the Tripartite theory in general, and what the current consensus is in the Japanese Historian community.