[Tuor of Gondolin]

Some comments from above
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"The orcs no longer existed as rational creatures."
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Is there some "canon" stating this. Since the orcs (derived from corrupted elves and/or men) were rational creatures, why would they no longer exist. After all,
they survived past decimations, from the First through the Third Ages.

Also, while it's a grey area, I don't think Rohan can be considered a
vassal state, but more an equal ally (see Aragorn's comments to Eomer after his coronation).

Another strength to Gondor in the Third Age would be a reoccupied Moria.
And indications in LOTR are that at least South Gondor was fairly heavily
populated before and during the War of the Ring. Faramir might have had
his hands full moderating a land rush into Ithilien (imagine the adjudications
over claims to ancestors lands abandoned earlier versus squattors in the land).

If Karen V's Atlas of Middle-earth is accurate, there would be significant lands (and presumably peoples) for disgruntled Easterlings to draw on, and eventually ally with resurgent orcs against (as observed above) a
dangerously overextended empire (especially in Arnor borderlands).