So, by that reasoning, Isaac and co shouldn't have had to save the world, they should have stuck to their mission and retrieved the elemental stars?
DD reflects the original two games in several ways. At the beginning of the Broken Seal, you weren't expecting that the retrieving the elemental stars missions would have turned into a mission to light the lighthouses and saving the world in a completely different way. Camelot was trying to emulate that by having a simple retrieval mission spiral into something greater. However, the problem with this is that Camelot is the producer, but the publisher, Nintendo, wanted that game ready by a specific deadline. As evidenced by the world map, the game was meant to be much longer, and it would have been much better should they had more time to complete it. But they were rushed and had to come up with an ending to the game that would have enticed players to actually continue playing until the end.
You see, you aren't thinking of the actual people behind it. It's obvious to see that it was rushed. I think the cliffhanger ending was another way of saying that the game was meant to be longer, but they simply needed more time to finish it. The blame should not be casted at Camelot, but rather at Nintendo.
Also, there is one word that I disagree with in your post specifically: that they could have done it easily. You obviously haven't coded anything, otherwise you'd know how difficult it is to make computers do what you want them to do. The easiest way to complete a game is to come up with a plan and run with it, because changing plans to something better halfway through is far more trouble than it's worth.