In regards to the deterioration of immortal cultivation and demise of immortals, there had been no reliable records of the happenings thus far. While there were many theories of what happened, most of them sounded nonsensical. None were the least bit related to historical facts.
The most plausible explanation was the exhaustion of spirit energy in the atmosphere, which caused the deterioration of immortal cultivation.
However, that just prevented the birth of more cultivators or immortals. During that period, immortals could be found in every corner of the world, and given their cultivation level, most of them should have been able to survive until today, just like Buddha himself, who was at least three thousand and seven hundred years old.