According to Zenos, the effect of tempering is essentially your will subjugating the will of another, in a way he tempered SHinryu by having a stronger will, which is very similar to what happened with Phoenix.
Personally I think they are trying to tell us something. They have shown us that primals can absorb echo blessed soul orbs and ascians souls.
Then they've shown us that primals can be tempered by the use of the resonance which is essentially a copy of an echo, with Zenos giving us a possible explenation to it.
It's possible that they are telling us they way to seal having ascians without reliying to hard contriviancies like the sword of light, which've seen to be very hard to conjure.

Otherwise I don't get why repeating this plot points.
Granted the reason of why summoning got widespread was because of ascians and would be extremely Ironic if their gift would be their ultimate end.

One thing for sure, it's about time that the story tackles the primal problem seriously.
So far we've been medication at best, we need to find a cure