It has a finger on the long term trend of decreasing relevance for labor and increasing relevance for capital as factors of production, but it's certainly not a metric I'd choose and that's why I tried so hard to steer towards something better.

One can imagine a world where productivity increases, the need for old jobs is reduced, but newer, better jobs more than replace them because the economy is experiencing genuine growth. Self-serving capital rhetoric will push you to always imagine it this way, self-serving labor rhetoric will push you to never imagine it this way, but good policy lies in figuring out what's actually happening in aggregate and responding accordingly (the framing I tried to push).

If productivity is increasing but not average salary, then by definition the additional wealth is being taken by the owners of capital.

It's not productivity itself; it's the decoupling of productivity from wages. If I'm creating 3 times as much value as my equivalent in 1970, why aren't I getting paid 3 times as much inflation-adjusted money, hmm? It's not even unfair to shareholders - they'd also get 3 times as much as in 1970. But instead they get 10 times as much and I get 0.7 times as much, or something like that. What's the deal?