I love the way you've written this so that it is incredibly open ended and easily able to be adapted to whatever sort of story the DM/GM or the players end up creating as a result of the sudden giants in the world. I could see a really heart wrenching scenario where the party tries desperately to save the giant they have bonded with as its coin winds down to the inevitable...maybe they find a way for it to exist in a smaller, more manageable and easy to maintain form?
I think the fact that it is just 'a moment' that starts suddenly with the giants' appearance and ends with their eventual but inevitable destruction, leaving all the larger questions and smaller details to whatever group happens to incorporate it is a hugely powerful device that makes it work beautifully as a Spark. The fact that the giants can be affected by the players, but will also autonomously just continue on their path and mutually assured destruction until none are left makes it feel like a real living event in a real living world.