I actually have a question about that. I can accept the idea that Kagrenac planned to ascend his entire race to a single god-machine. It fits the character. However, does that necessarily mean he succeeded? If so, why the Mantella, and how would Akulakhan have worked with the heart of Lorkhan? Just help me out here, because it's a fun idea and I'm curious.
Also, in some endings Numidium is destroyed. Does that mean they're really gone if they did succeed?
The Mantella was a replacement power source/stone/whatever when Vivec did not give Tiber Septim the Heart. It probably would have worked a lot better with the Heart, because it was made for it. Unfortunately for the Dwemer, in the final stages of Kagrenac's plan, the Chimer attacked, leading to Kagrenac to carrying out the ascension prematurely, neglecting to put the Heart in Numidium before ascending his entire race into Numidium's skin.