Sometimes it isn't about the ending. Occasionally it is worthwhile traveling down a path to a known destination; a path that is filled with hardship, moral lessons, discovery, pain, and growth. The ultimate futility of the act falls within the realm of philosophical pandering like "Why bother with anything?..."
The reason is that we grow through firsthand experience. And, should the game deliver on its premise, you will get to watch a planet die from ground level. How utterly awesome is that prospect? Regardless of feeling cheated out of a 'good' ending, think on this:
You've repelled waves of enemy forces that would have taken out planetary garrisons. Your teammates have been dying like heroes of legend, you have suffered more in these days than ever before, but you have bled the enemy unlike any force they have known. And then, on the seventh day of the seventh month, just as the tide breaks and you catch a breath at last, you see the storm clouds gather. The enemy forces flee in panic, the surviving few of your warriors remove their head cover, and you stare solemnly, arms lowered, as everything you fought for is bathed in strobing light, becomes engulfed in flame, and dies ignominiously, flattened to endless fields of lifeless glass...
And then a 'restart at last checkpoint' prompts appears... Kidding!
If you cannot derive satisfaction of a defiant victory, of bravery in the face of fate, the ageless triumph in the myths of future generations, both yours and the enemy...
Well, then you must not have read nor seen 300 and other works like it. Sure, everyone dies... But it's the effort that counted.