Measuring aircraft production per weight doesnt sound like that bad of a proxy. If I hear that Boeing produced 300 kilotons worth of aircrafts more this year, I'd be right to suspect they've ramped up production.
I think you're overlooking the lesson of Goodhart's law: you can use a metric, but if you make it a target, it stops being a good metric. Neither "tons of aircraft" nor "lines of code" should be the measuring stick -- and if they're not, then they can still be used as metrics.
To be fair, the hazard with AI agents is that they generate fluent output that is often facile, so it's easy to do a lot of things while having a lot of defects. That's a sign that quality control is not prioritized enough. A change in quality will also reduce the utility of SLOC as a metric, but the mechanism is different than what Charles Good hart pointed out.