Well that's a matter of perspective. He was the first one of the Avengers to be a hero and establish the notion of the Avengers. Its like Superboy and the Legion. He inspired them. Of course technically speaking, sure. He was the "last" "original". Whether he is technically the "first", "last", or "sixth" he's obviously always been the symbolic inspirational force behind the Avengers.
Of course who is to say that's the way the story will go in the film? In Ultimates Cap is in fact the first guy which Fury and Shield based the idea of the Ultimates around. They had Pym and Stark and Banner working on the idea before then but it was the discovery of Cap that made the idea real. I think. Its been awhile since I read it.
I also think it undermines the idea of him being an Invader and the whole team in general too. It's like "so, these guys were all fighting the war with their powers (or just general awesome skills), but who fucking cares about that? AVENGERS!"