If it's anything like redesign work I'm used to, someone redesigned and just happened to remove it with little reason for it. Then the numbers went up as people had to scroll so they assumed everyone loved the redesign and didn't question why.

I think this hints at a concept I find very hard to articulate about letting metrics guide your design. You set out with metrics that seem plausible, you work to improve them, and if they do improve, you call the design a success. But then it actually turns out the metrics were no good in the first place because they “improve” for scenarios where you’ve most certainly made the user experience worse. Metrics proponents would argue that you should have had better metrics, but then I start to feel that the problem of selecting the right metrics becomes an art form just as subjective and complicated as designing a good UI in the first place, at which point they’re no help.

Goodheart’s law may not precisely capture what you’re getting at, but seems closely related

https://en.m.wikipedia.org/wiki/Goodhart%27s_law

Engagement is a nuanced metric for that reason. Am I engaged because I am enjoying the product or is it just taking me longer to achieve things because the UX is bad.

> selecting the right metrics becomes an art form just as subjective and complicated as designing a good UI in the first place

this is absolutely the truth!

I would say that perhaps what is lacking is a clear vision of the product. Steve jobs didn't use metrics for the initial design of the iphone.

I suspect that as a company grows bigger and bigger, the product loses the initial visionary that created the success in the first place. May be the "founder" (of the product, not necessarily of the actual company) left, or moved on, and what remains is just diluted responsibility between a few different people. Therefore, they resort to metrics, as they have no idea what works, nor have a vision.

Knowing Google, it was probably a feature that someone didn’t think was important enough in a UI refresh to spend time on and was added now because the team happened to have time (or the interns have started).

That wouldn't make sense. YouTube wants you to watch ads, not click around the UX.

Time spent scrolling is not time spent watching ads. So that is definitely not a metric YouTube is optimizing for.

I have no doubt that total time spent in the app is a metric they track. Plus scrolling through all of those videos on the way to the video you're actually looking for means there's a decent chance you'll see a thumbnail or title that catches your interest leading to more time watching videos. Either way, It'd be a shitty move and one that somebody should have caught and corrected a long long time ago.

I imagine the cost of the index used to store videos by oldest uploaded outweighed the likely marginal number of users who used it?