> There’s no proxy for content quality.
I don't think that's true, it's just that the proxies are imperfect. But the specific alternative metric wasn't that important to my point.
> Engagement in contrast, is simple, reliable and measurable.
That's the root of the problem: the algorithms have been designed to use "simple, reliable, and [easily] measurable" things as their whole input, and then narrowly A/B tested to maximize some other easily measurable output. Looked at holistically, it's too simple to do an acceptable job.
If you took a similar attitude, and applied it to create an algorithm for another discipline, like medicine for instance, it'd be pretty clear it'd be a disaster.