You've surpassed the bounds of the story and are now just using assumptions to support your assumptions. Hagoromo was never stated to develop techniques with Creation Of All Things and he has no reason to develop a technique that seals souls. The Uzumaki came from the Senju and yet they have had no connection to the Reaper Death Seal in the story, so certainly such a thing was never handed down to Ashura. Not every technique in the story in some derivate of what Hagoromo or Indra did.
Certainly I have no intention of believing such assumptions to support something that is already an assumption.
Not really the Senju had no combat use for it different senju specialised into different fields eventually splitting of its natural for a group to lose kbowedge of something. You have to remeber this is 1000 year process.
Anywa my assumptions are based upon rational evaluation of mangafacts. Yours can't even explain 9 billion + seal combos mine gives at least some explanation for them.
The spiral seal is an uzumaki seal yet kurama recognises it once Naruto takes his chakra and reseal kurama with it as the sage of 6 paths seal. Kushina claims it's one of theirs I didn't here of the Senju using it or having knowledge of it. This adds weight to my theory of specialisation and split off.