Entwives

[Orald]

In Letter 144 Tolkien seems to think the entwives were pretty much gone for good:

I think that in fact the Entwives had disappeared for good, being destroyed with their gardens in the War of the Last Alliance (Second Age 3429-3441) when Sauron pursued a scorched earth policy and burned their land against the advance of the Allies down the Anduin (vol. II p. 79 refers to it2). They survived only in the 'agriculture' transmitted to Men (and Hobbits). Some, of course, may have fled east, or even have become enslaved: tyrants even in such tales must have an economic and agricultural background to their soldiers and metal-workers. If any survived so, they would indeed be far estranged from the Ents, and any rapprochement would be difficult – unless experience of industrialized and militarized agriculture had made them a little more anarchic. I hope so. I don't know.

Ents themselves would probably not become extinct because they are basically just trees. Treebeard says this in more than a few words when he tells Merry and Pippin that trees are always stirring, becoming entish and that many Ents become treeish. This explains the decreasing number of older Ents, and the trees in the Old Forest.

If you want to take a look at what Treebeard has to say I suggest you pick up a copy of TTT and read the chapter entitled 'Treebeard.'