The story mentions how Ice Heart wields the power of the Echo, and how the Echo allowed her to retain consciousness while embodying the primal Shiva.

Wouldn't it be cool if Summoners instead transformed into Demi-Primals and gained new skill sets for each? (Let's be honest the Egis are real dumb.)

It sounds like what Ice Heart did is what Summoners in theory should be doing; channeling the power of the Echo to wield the power of a Primal.