Sort of, but you're missing a couple of things.

Firstly, if miners' revenues overall decrease, so does the amount of miners.  The difficulty adjusts regularly to allow this to happen, so therefore mining is automatically profitable to some extent.

Secondly, the block reward is denominated in Bitcoin, not fiat currency obviously.  Therefore if the Bitcoin price doubles each four years (which it's likely to do for at least a couple more block halvings) miners' revenues will stay the same or more in terms of actual value from the block reward.