Can't remember where this was mentioned, or who it was (John or Christopher) but I've read a paragraph somewhere, possibly in the Letters, that entertained the idea that Middle-Earth passed into the fourth age and then the fifth and so on until it reached a point in which it was similar to our modern world.

You can find information about Ardas seventh age which correlates to our modern times, starting with 7th age year 1 being being the start the Christian "AD" time reckoning. Basically Ilúvatar enters Arda the same way God is said to have entered the world as Jesus.

All this information might be from drafts never officially published or similar however, I can't remember. It was quite long ago I read about it.

Edit: after searching a bit online I found these references; [The Nature of Middle-Earth, "Part One. Time and Ageing: VI. The Awaking of the Quendi", Text A, p. 39] and [Morgoth's Ring, "Part Four. Athrabeth Finrod ah Andreth: [Draft material]", p. 356] if you are interested to read about what is actually said about it.

Also check out this Wiki page:

https://lotr.fandom.com/wiki/Seventh_Age?so=search

Seventh AgeThe One Wiki to Rule Them All