Electro101

Ah... That's why she wanted Legion 7.

And to answer your question, she knows Legion would betray her (yet again, if the gains outweigh the risks). Remember the chapter where Legion 8 had the opportunity to acquire mother high heaven's realm heart in the battle between the dragon apparition and mother high heaven. Legion didn't go through with it because, at that time, Legion was too weak for that battle and acquiesced.

The risks outweigh the gains, so Legion rejects, but if the gains outweighed the benefits, Legion would have accepted. Mother High Heaven believed that they would have progressed far enough for the Realm Lord to not pose as a significant threat to Legion. She gambled, and she failed. Simple as that. If Legion were to have grown enough, the rewards of capturing the realm lord would have outweighed the risks, and they would have accepted. She didn't come empty handed, she came with the incentive of rewards that she deemed would have been enough if they had grown faster, but they have not.

She didn't forget. She believed that with time they would have grown enough to go against the realm lord before she needed to resort to this and also had benefits to bring with this offer, and that would be the rewards for capturing the realm lord. No where does it say that she gambled on them for sentimentality. Legion would be willing to gamble as well if the venefits outwayed the risks, but in this case, they don't.