The wealthy getting wealthier isn't necessary a problem, as long as the same effect exists in the middle and lower classes as well.
If, on the other hand, the middle and lower classes are getting comparably poorer as time goes instead, your society are in huge problems.
No nation to my knowledge, survived intact the above scenario for more than 1-2 generations in a row. After 2 generations -> rebellion, civil war, war, terrorism, economic meltdown, a nation divided or destroyed.
The middle class and it's well being, is any modern nations backbone, carrying the nation and its populace forward. Always have been.
