The idea seems not wholly inconsistent with Professor Tolkien's view of the declining moral fibre of Men as the Fourth Age wore on, although in connection with
Boromir88's post I think it would only be likely in a time when the events of the War of the Ring and so on had begun to pass into truly ancient history. Nonetheless I can imagine Gondor and Rohan becoming fractious with each other, perhaps even because of the Oath of Eorl and the gifting of Calenardhon. I could imagine those becoming a source of resentment on one side or another in time, when the memory of the honour and dignity of the friendship between the two kingdoms had been forgotten.
The Oath refers particularly to the mutual friendship between the kingdoms against "the Shadow in the East" and "the Shadow" in general; I can imagine in time arguments being raised as the realms became more 'political' that, with the defeat of the Dark Lord, the Oath no longer carried the same weight.
It's a depressing concept to imagine, but unfortunately I don't think it would necessarily be out of place thematically
