In the short term it is - when grid parity is reached, there will be more demand than suppliers will be able to fulfill, and there will be no reason to go lower than just below the cost of traditional electricity. Additionally, it's quite likely that for the first couple of years only a limited number of players will have the tech to produce/deploy at the lowest prices, and as such will effectively (for a while) have a monopoly with no incentives to compete on price.

Of course after a few years these situations will change and as more players enter the market (lured in by the high profits of the existing ones...), the downward trend will continue, without the anchoring to 'grid parity' that exists today.