Fortunately we can still talk about apples and pears and bananas as "fruit" without that meaning seen as no longer current. (Currants too for that matter.)

I think there are enough examples of gay being used in its carefree/happy etc sense in our literary and song heritage for it still to survive and even revive. After all "gay" in it sexual orientation sense got the positive connotations that made it useful because of those associations.