beta
Zeno sealed Jae-ha, Kija, and Shin-ah not because it's the end of their lifespan. That chalice is really important to Zeno not just because it's what gave them the powers, but it can actually extend their lifespan. There are a few theory I consider about this,
1. Zeno sealed the dragons so they can recover, both their souls and their bodies.
2. Three of the four dragons aren't going to die young this time around because the reincarnation of King Hiryuu has descended. The first generation of the dragons all lived until old age, the descendants died because King Hiryuu wasn't in their generation(s).
3. Zeno would release them back when they're ready or when the time has come for their release.
4. As Kija felt, Jae-ha and Shin-ah are still alive, it made no sense that the three dragons are dead or waiting to be reincarnated again because Yona would sense it. Zeno looked relieved because he sealed them before they died or lost themselves.
Again, this is just my theory. I hope it's not too far off.