My opinions are certainly different from anyone else’s that I have ever heard including those posted here.
I believe that the plan of God did not fail. Those who were planned to exist from the foundation of the world will be saved. Those whose existence comes into effect because of the fall still can be saved but in reality they will not because they actually belong to someone else.
Certainly I could be wrong but it is something to consider.
Here are some Scriptures.
Her conception is greatly multiplied,
Ge 3:16 Unto the woman he said, I will greatly multiply thy sorrow and thy conception; in sorrow thou shalt bring forth children; and thy desire shall be to thy husband, and he shall rule over thee.
Even though Cain was of that wicked one God still tried to show him the way to life.
Ge 4:6 And the LORD said unto Cain, Why art thou wroth? and why is thy countenance fallen?
7 If thou doest well, shalt thou not be accepted? and if thou doest not well, sin lieth at the door. And unto thee shall be his desire, and thou shalt rule over him.
1Jo 3:12 Not as Cain, who was of that wicked one, and slew his brother. And wherefore slew he him? Because his own works were evil, and his brother’s righteous.
Also here is another example of God reaching out to those who will come up short,
Mt 23:37 O Jerusalem, Jerusalem, thou that killest the prophets, and stonest them which are sent unto thee, how often would I have gathered thy children together, even as a hen gathereth her chickens under her wings, and ye would not!
Joh 8:44 Ye are of your father the devil, and the lusts of your father ye will do. He was a murderer from the beginning, and abode not in the truth, because there is no truth in him. When he speaketh a lie, he speaketh of his own: for he is a liar, and the father of it.
Notice “from the foundation of the world”. And the reactions of those who were not: how they react in certain ways.
Eph 1:4 According as he hath chosen us in him before the foundation of the world, that we should be holy and without blame before him in love:
Re 13:8 And all that dwell upon the earth shall worship him, whose names are not written in the book of life of the Lamb slain from the foundation of the world.
Re 17:8 The beast that thou sawest was, and is not; and shall ascend out of the bottomless pit, and go into perdition: and they that dwell on the earth shall wonder, whose names were not written in the book of life from the foundation of the world, when they behold the beast that was, and is not, and yet is.