On the contrary I feel the evidence is pretty much conclusive. arigatō comes from arigataku (an adverbial form), through a well-known "u-sound" change, as also

On the contrary I feel the evidence is pretty much conclusive. arigatō comes from arigataku (an adverbial form), through a well-known "u-sound" change, as also explained on the page. Influence from Portuguese was not needed for that to happen. (And according to this page: https://selftaughtjapanese.com/2015/11/02/the-real-origin-of-arigatou-%E3%81%82%E3%82%8A%E3%81%8C%E3%81%A8%E3%81%86-japanese-for-thank-you/ https://selftaughtjapanese.com/2015/11/02/the-real-origin-of... the word was used in its "ou" form before the Portuguese even arrived in Japan - 1543)