The footnote in the first sentence of the last-quoted excerpt offers a fascinating insight:
(*) I imagine the gap to be about 6000 years: that is we are now at the end of the Fifth Age, if the Ages were of about the same length as S.A. and T.A. But they have, I think, quickened; and I imagine we are actually at the end of the Sixth Age, or in the Seventh. [The Letters of J.R.R. Tolkien, 283 (#211)]
Tolkien would have been aware of Archbishop Ussher's calculation of the date of Biblical Creation being the year 4004 BC. This is about 6000 years ago which matches Tolkien's imagined gap between the Fall of Barad-dur and our own days.
Although some modern Christian fundamentalists consider that Ussher's date is the actual date of Creation, Tolkien was not a fundamentalist and would not have believed that the date was literally true. He was, however, a faithful Roman Catholic and perhaps, consciously or subconsciously, he dated his literary creation to a time prior to the supposed date of the Biblical Creation. Therefore, the change in the Earth which altered geography from Middle Earth to Modern Earth may have been caused by the "Biblical Creation" which usshered (pun intended!) in Tolkien's own personal religious beliefs in place of his imagined ones.
