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The average cost changes over time, and if it has to trade at above the current average cost it already would be.

And there are real reasons it could sustainably trade below cost to mine, e.g., if mining is concentrated in actors with exogenous interest in the ecosystem, such as sellers of goods whose sale is facilitated by bitcoin rather than fiat being the medium of exchange.

Also, the Bitcoin received as mining rewards might sometimes be more valuable to the recipient than the notionally greater amount of other currency (or even nominally greater amount of Bitcoin!) spent on equipment and power, eg if it is outside of sanctions, no longer tied to proceeds of a crime, no longer connected to a state's budget, etc.

They can just turn off their asics when they mine at a loss

Or at some point, some miners will have to call it quits...