I'll have to agree that is a rather retarded demo concept especially considering you don't really get a feel for the whole battle system or really any look into the overarching story. I get the feeling that's only a small portion of the game (if it's even in the game, for all purposes this could of just been a proof of concept demo (a shitty one)). The Iron Giant battle looked sweet (though I still don't like the combat system). I know the first demo went over well with fans and it was an actual better look at game mechanics.
Honestly I agree with you on breaking the plot up and spreading it across different formats, it's retarded and is only a money making ploy (something SE is becoming a master of doing lately). Plus it dumbs games down and forces people to go above and beyond just to get the whole story experience. As far as I know the anime (5 ONA episodes) is a prequel to the actual events of the game and helps set up the world and continuity (I took that almost word for word from the Wiki page on it). Honestly you could probably skip watching it and not really miss anything since, as FF stories usually do, they'll spill almost all past story elements in some sort of long dialog/flash back sequence. I also don't think they'll split this game in multiple titles. Not only has the game been in development for 10 years (originally starting off as a much more interesting game called FF versus XIII with a completely different concept) it wouldn't be financially sound to split a game like this into parts (considering the debacle with the FFXIII games, that while they sold well most received poor reviews). I could be wrong, we are talking about SE and they haven't been making financially sound decisions since FFXII and the complete rework of FFXIV.
Its sad to say but I expect we'll be seeing this a lot more in the future from SE with follow-up FF games. FFVII will probably be the first in a jaunt down memory lane and I can see remakes coming for various other titles in the series (we've already seen a remake of FFIV and a sequel to it so I wouldn't be surprised if we get a remake of FFVI and a remaster of FFXII). I don't honestly expect to see a main franchise game coming out for a few years now and that will all be based on how well XV does and how well it's received (FFXIV we'll be SE's new XI and I'm sure it'll live on for quite a few years before they drop it). I think SE is putting all their hopes in XV doing well and if it doesn't that the FFVII remake will help fans forget it if it flops (though the FFVII remake is it's own can of worms and based on their current track record I forsee a lot of pissed off fans in the future).