It's more complicated than being left behind. Knowing what I know now, if Thrawn hadn't sent me away I would've probably died during the Galactic Civil War. There was nowhere in the Empire for me to develop the skills he taught me.

Sure, there were probably other factors in there. Investing time training someone only to have them die stupidly would be frustrating for him to see.

If Elias makes that choice one day, it may be due to reasons that we can't see. An attempt to do something good without taking into account emotions.

[But yeah, Chise is right. Eli feels left behind and that hurts.]