Elves represent mankind in an "unfallen state", as pointed out by the author. Thats us - still walking around naked in Eden (omg thats an ugly mental picture isnt it??). In some ways, he treats the "gift of men" as the same as the bible treats the gift of the apple. "Men are weak and elves are not" is (IMO) a weak premise
Its the type of weakness that counts. In LOTR, the weakness referred to men is primarily the desire for rule or power over others, and the little (or lack of) moral temprament that was invlolved in achieving said power.... right?
By the 3rd age - yea there werent a lot of elves (particularly Noldorian) around who had that particular condition. But there were plenty in the 1st age and to a lesser extent the 2nd age as well. Desire for power, ego, hubris - you name it.
Would "resistance to change" be considered a weakness by the 3rd age? Did that help, hinder, or have no effect on the big mission of the time?