There are many lost and revived teachings in Buddhism. Since how and when teachings survive or are reborn is a complex religious and social matter, I don't consider it as an accurate measurement of the quality of any doctrine. The variety of teachings is also not a problem nor an issue of superiority, just as the Buddha gave many methods in order to lead everyone to liberation so are there many ways one can choose from freely even today.
As for instructions of abiding in the unborn, it can be put into modern everyday terms to make it clear to anyone. Abiding in the unborn means not giving birth to fixed ideas about whatever we experience. It is called unborn because it is without any concrete view, concept or feeling. Once a label that we believe to be real is attached to something there will be emotions and from emotions deeds, habits and new births. But if we understand that only the unborn is our true nature and not any thing that the mind gives birth to, there is no reason from then on to identify with passing experiences. Of course, the teachings of emptiness and selflessness are the same as this, there is nothing new invented, only the format changes.
In Bankei's words:
"That you do see and hear and smell in this way without giv-ing rise to the thought that you will is the proof that this in-herent Buddha-mind is unborn and possessed of a wonderful illuminative wisdom. The Unborn manifests itself in the thought "I want to see" or "I want to hear" not being born. When a dog howls, even if ten million people said in chorus that it was the sound of a crow cawing, I doubt if you'd be convinced. It's highly unlikely there would be any way they could delude you into believing what they said. That's owing to the marvelous awareness and unbornness of your Buddha-mind. The reason I say it's in the "Unborn" that you see and hear in this way is because the mind doesn't give "birth" to any thought or inclination to see or hear. Therefore it is unborn. Being unborn, it's also undying: It's not possible for what is not born to perish. This is the sense in which I say that all people have an unborn Buddha-mind."
(Waddell: The Unborn, p. 88)
Regarding Bankei's arduous practice, he explains:
"You can gather from what I've told you that my practice lasted many long years and that I came to realize my Buddha-mind only after great hardship. But you can grasp your Buddha-minds very easily, right where you sit, without that long, punishing practice. That shows the relation that links you to Buddhahood is stronger than mine was. You're all very fortunate indeed."
1 Myriad dharmas are only mind.
Mind is unobtainable.
What is there to seek?
2 If the Buddha-Nature is seen,
there will be no seeing of a nature in any thing.
3 Neither cultivation nor seated meditation —
this is the pure Chan of Tathagata.
4 With sudden enlightenment to Tathagata Chan,
the six paramitas and myriad means
are complete within that essence.
1 Huangbo, T2012Ap381c1 2 Nirvana Sutra, T374p521b3; tr. Yamamoto 3 Mazu, X1321p3b23; tr. J. Jia 4 Yongjia, T2014p395c14; tr. from "The Sword of Wisdom"