When we reach that point, short of a repeal of the law of supply and demand, the scarcity of labor will almost surely induce a rise in hourly compensation gains that increasingly outpaces an even faster productivity growth - a condition that would cause unit costs to accelerate over time.
Moreover, we do not know how long net imports and US external debt can rise before foreign investors become reluctant to continue to add to their portfolios of claims against the United States. At that point, the safety valve of net imports could narrow or close.