I don't see why the 'amount of Gods per person' is a valid metric for anything. Progression from poly- to monotheism doesn't imply a future progression to atheism.

The actual percent of atheists in society has indeed increased over time, but it's never been significantly above 10% worldwide and it's not clear that's it's rising right now (Wikipedia source). It's hardly strong enough evidence to conclude that a majority of humanity will be atheistic one day. Other religions surely exhibit or previously exhibited rising trends at least as strong.