He is not just saying useless words for encouragement. Tesrat has become older but she still has at least 50 years to try and coerce her cells to switch to metabolizing Mana for vitality. She is still alive and it is better than what some get. Sauron has been fighting a lot of wars these years so a lot of warriors from the original Iron Fur pack have died. It is a boon that she still has her life ahead of her.
"Yes, I suppose you're right. But what are the chances of me becoming an Alpha of Alphas?" She asked.
He replied, "It is very high if you ask me. You have shown progression all these years. You haven't stagnated so there is still hope for you."