This is especially discussed in light of his other controversial claim, that the life of the Dharma would be cut in half when he began ordaining women.
But all of the academic speculation seems quite beside the point as I read today in the Rhys-Davids and Stede Pali Dictionary that:
Note. When Gotama said that his "religion" would last 500 years he meant that it would last a very long time, practically for ever. The later change of 500 to 5,000 is immaterial to the meaning of the expression, it only indicates a later period (cp. 5,000 in Nibelungeniled for 500, also 5,000 men in ambush Joshua 8. 12; converted by Peter Acts 4. 4; fed by Christ with 5 loaves Matthew 14. 21). Still more impressive than 500 is the expression 5 Koṭis (5 times 100,000 or 10 million), which belongs to a comparatively later period, e. g. at DhAI've read several accounts showing that the number 84 (84,000, 84,000,000, etc) is similarly of mythical significance in ancient India. Likewise, in a recent Journal of Buddhist Ethics paper, Charles Prebish points out that the number 100 was probably at least at times used as a mere approximation.
i.62 (ariya -- sāvaka -- koṭiyo), 256 (˚mattā -- ariyasāvakā); iv.190 (p. koti -- mattā ariya -- sāvakā).
But this is the first I had heard of a similar clarification in regard to the 500/5000 years issue. Very interesting...
* image from Kit's Bits.