By that standard, I suppose we should be grateful for smallpox too...
Putting that aside, everything you have said easily applies to polytheism too.
In terms of applying gratefulness and yielding happiness, monotheism is best because it yields the most happiness. Whereas polytheism convolutes this principle with the number and dimension of deities, monotheism is simple and straightforward, not to mention easier to pass down generationally. I would also assume monotheism is the more peaceable of the two because it suggests unity. Proponents of polytheism are liable to divide themselves based on their beliefs and conflict with each other.
I would disagree that "everything I have said easily applies to polytheism too". If you considered the two objectively and in terms of gratefulness yielding happiness, which would you consider the most efficient? Is it polytheism or monotheism which best maximizes gratefulness?
So far as being grateful for afflictions, I did not say that gratefulness could not accompany grief. An example is when a loved one suffers from an illness and dies. The family feels grief at their loss but is grateful their loved one no longer suffers.
If you still think what I said about monotheism can easily apply to polytheism, would you care to elaborate?