Although it is a horribly pessimistic thing to say, doesn’t the fact that this preserved line of Buddhist thought, “the dharma will only last Y amount of time,” which is abundant in early Buddhists texts, Mahāyāna texts, and Tantras alike, the fact that it runs to contrary to the thrust of Buddhism as a “lasting” world-historical religion, doesn’t that make it seem slightly more likely to have been an actual remembrance of something the Buddha said?
Jesus didn’t say his Church would only last 1000 years, he didn’t say it would only last 5000 years.
Mohammed didn’t say the Qu’ran would only remain unaltered for any number of years.
The Pope doesn’t teach that the Catholic Church will only last 200 or whatever more years.
But if they did, and it was remembered, and it wasn’t forged slander, then 4000 years later you could look at this memory of Jesus, or Mohammed, or the Pope, and be suspicious that the memory was likely accurate. Because they would not have said that in a context where their original words were lost or forgotten. Their word-compilers would have only compiled the words that aren’t problematic in such a way.