They can look at how long people watch; but that ends up rewarding rambling pointless blather when a concise two minute video would be perfect.
They can look at likes or subscribes, but that causes many wasted human lifetimes per day, of people saying and listening to pleading to click said buttons.
Perhaps, but only because Google is being small minded. Define the "problem" in a certain way, and come up with certain solutions. A broader minded frame would define the problem(s) more broadly. Presenting users with UI. Creating a good, or at least respectful, commercial incentives framework for creators. The litmus for this is the content. What content gets created. Mitigating or avoiding bad consequences, like spiraling towards pointless blather.
Yes, this would require subjective decision making, but at least it doesn't require dim witted, corporate delusion. Call the dog by its name.