Frankly, I don't think you understand Japanese morphology at all and are blowing a whole load of hot air.

The inflection of arigatashi -> arigatou is quite straightforward and any adjective in Japanese can undergo the same inflection:

- arigatashi -> arigatai: modernization

- arigatai -> arigataku: adverbalization

- arigataku -> arigatau -> arigatou: u-onbin

It would perhaps also help your case if you didn't misspell "arigatou" twice in your post, either.