Do you ever wonder why Jigoro trained Kaigaku and Zenitsu and send them to be demon slayers despite experiencing how terrific and life-threatening the demons' battlefield is? I think he didnt trained them just so he had a successor.

I believe that he trained so that they know that they do have a worth. When he took them in, he saw the helplessness and hopelessness in their eyes. The way their eyes scream that they don't have any sort of worth in them. That their lives are meaningless. He doesn't want them+

+to think themselves that way. That is why he trained them. He have his utmost faith in them that they WILL survive till the end, that they WILL able to protect people and slay demons even if they couldn't able to master all the forms.

trained them*