In order to understand the context and what people in Theravada Buddhist countries believe, we should be familiar with a story in a post-canonical Pali text called the Anagatavamsa which has been composed several centuries ago and it reflects the way of thinking at that time (assuming that nobody can realize Nibbana at present anymore and we should perform meritorious acts while waiting for the next Buddha to appear):
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[Before the coming of the Maitreya, the holy religion will decline.] How will it occur? After my decease, first will occur the five disappearances. And what are the five disappearances? The disappearance of the attainments, the disappearance of the method, the disappearance of learning, the disappearance of the symbols, the disappearance of the relics. These are the five disappearances that are to occur.
The attainments: Only for a thousand years from the time the Blessed One passes into Nirvana will the priests be able to acquire the analytical sciences. Then as time goes on my disciples will attain only to never-returning, to once returning, to conversion. As long as such exist the disappearance of the attainments will not yet have occurred. But with the death of the last disciple that has attained to conversion, the attainments will have disappeared. This, O Shariputta, is the disappearance of the attainments.
Disappearance of the method: My disciples being unable to realize the trances, the insights, the Paths, and the Fruits, will keep only the four purities of conduct. Then as time goes on they will keep only the commandments forbidding the four deadly sins. As long as there are a hundred or a thousand priests who keep the commandments forbidding the four deadly sins, the disappearance of the method will not have occurred.
But when the last priest shall break the precepts, or shall die, the method will have disappeared. This, O Shariputta, is the disappearance of the method.
Disappearance of learning: As long as the text of the Three Baskets, which is the word of the Buddha, and as long as their commentaries are extant, the disappearance of learning will not have occurred. But as time goes on there will be irreligious kings of base extraction, and the courtiers and others in authority will be irreligious, and then the country people throughout the kingdom will be irreligious. On account of their irreligion the god will not rain in due season, and the crops will not flourish properly. And when the crops do not flourish, those who are wont to give the reliances to the congregation of the priests will be unable to do so any more. And the priests, not receiving the reliances, will not teach the novices, and as time goes on learning will disappear.... [and likewise with the disappearance of the symbols]
'Then when the Dispensation of the Perfect Buddha is 5,000 years old, the relics, not receiving reverence and honour, will go to places where they can receive them. As time goes on and on there will not be reverence and honour for them in every place. At the time when the Dispensation is falling into (oblivion), all the relics, coming from every place: from the abode of serpents and the deva-world and the Brahma-world, having gathered together in the space round the great Bo-tree, having made a Buddha-image, and having performed a "miracle" like the Twin-miracle, will teach Dhamma. No human being will be found at that place. All the devas of the ten-thousand world system, gathered together, will hear Dhamma and many thousands of them will attain to Dhamma. And these will cry aloud, saying: "Behold, devatas, a week from today our One of the Ten Powers will attain complete Nirvana." They will weep, saying: "Henceforth there will be darkness for us." Then the relics, producing the condition of heat, will burn up that image leaving no remainder.
'This, Sariputta, is called the disappearance of the relics.'