When translating a work from page to screen there should be an obligation to be true to the original work.
If you want to change the story, then don't use the same title because you are not telling the same tale.
They make the same basic point and accomplish the same goals in a way that's consistent with already established characters and events in the story without introducing a completely new (and bizarre to the uninitiated) element out of the blue.
Plus, they improve on the dramatic weight of the ending by having Dr. Manhattan make the ultimate sacrifice for the good of mankind.
I'm not seeing the big issue here, beyond a demand for ridiculous levels of faithfulness to the story that simply can't always be done when translating to film.
I think this ending is a vast improvement over the original, and frankly makes more sense within the context of the story. Much of the story is about Manhattan's alienation from humankind and eventual decision that they are worth saving, and instead of capitalizing on that more, they bring in the Squid. This actually ties in better with what came before, and gives a plausible reason for the actual cause of the escalation between the US and USSR to leave forever.