The thing with XIII is that is was not a success, there was nobody asking for a sequel, if they didn't plan a sequel from the start then why make one? You say it's to use the resources they already had but that doesn't convince me once bit, I think they already knew there were going to be 3 games from the very start. If it's just for resources then why does this third game sound and look like it's made from scratch? It's a totally different world and everything, if it were just to use resources, would they not continue to use them? I don't buy it. They knew there was going to be more than one XIII game, hence why we were always introduced to the XIII games as a series of games, instead of one game.
They introduced XIII as part of fabula nova crystallis, a series of spin offs relating to a mythology with each their own story related. initially with a "XIII" on it, however was dropped when Agito XIII was renamed "Type-0". We just recently heard the term "Lightning Saga"
Also, there are often times when a sequel isn't asked but they make one anyways...it has nothing to do with demand of a sequel. Fans never asked for a se