[JeffF.]

Tolkien wrote in the Silmarillion about the host of the Last Alliance of Men and Elves when it gathered at Imladris (Rivendell) that, "none greater has been mustered (in Middle Earth) since the host of the Valar went against Thangorodrim." Given that the kingdom of Arnor was only around a hundred years old the Eldar must have numbered a fairly large part of that host (which was smaller than the army that actually fought at Dagorlad since the Silvan Elves and the Army of Gondor joined it after the gathering at Imladris). I grant that most of these troops at Imladris must have been men, Tolkien writes that he Eldar dwelt most in the lands beyond the Ered Luin, in the Havens and in Imladris implying that most of the lands of Arnor must have been occupied by men under Gil-Galad's rule (prior to him ceding these lands and people to Elendil).

In reading the Peoples of Middle Earth book I agree that the Eldar were diminishing but it seems that Tolkien meant mostly in stature (he calls the Noldor Gnomes in Peoples of Middle Earth) making statements implying they were shorter than their ancestors.

The affect of the "weariness" seems to be limited to the Eldar, the Silvan Elves seem to be much as they have always been, preferring to keep to themselves and only intervening during the in a limited fashion in the war on Sauron, their hosts in the Last Alliance being the last time they march on the Dark Lord. The Avari also seem immune to the "weariness" though of course they are almost a non-entity as far as the history of Middle Earth goes.