Miners make up a portion of the sell side of the BTC-fiat markets, and yes, they need to sell at a price that exceeds their cost to obtain their bitcoins. But to say that the overall market will converge to that metric is to ignore all of the non-miners participants in it.

As more bitcoin gets distributed to non-miners, a growing proportion of the market will set prices that are completely removed from the miners' marginal cost of production. Most market participants won't even know anything about the cost of production. Buyers and sellers will set prices for their own reasons, and miners will make up a smaller and smaller portion of that action.

The price of bitcoin in fiat terms will be the aggregation of all participants' calculations, and that will likely not be at all correlated to the marginal cost of production. Miners time holding or selling or even buying based on market moves that are mostly out of their control, just like everyone else.