Why is ionization always an endothermic process?

My textbook says removing an electron always requires an input of heat. That makes sense because of coulombic force between electron and nucleus. But it also says that the process is endothermic always.

Why is that? In the case of Na, after the input of "activation energy", isn’t Na+ more stable than Na? So therefore it would be exothermic?

Furthermore, this is not always true for electron affinity like it is for ionization. Adding an electron to Helium is an endothermic process because He- is less stable than He.* But it’s exothermic for Cl and Cl-.

*Not totally sure about that. Contradicting information on the internet. Some sources say electron affinity of He is 0. Others say its negative (thus endothermic).