The page has since changed again, I believe, and it's even more of a mess. Scanning the page while scrolling is nearly impossible unless you've very carefully placed your mouse pointer over elements that do not expand on hover. (It's really maddening and I barely use the page anymore because of it.)
I may be wrong. To confirm whether things changed or not, we'd need to get Google insiders to speak out on that subject.
There is another possibility, that direct (verbal) user feedback is discounted as being "anecdotal" and not included when making "data driven" design decisions. But as you mentioned, this is all speculation without an insider's POV.
My intuition, however, is that their data-driven technique over-optimizes the various minutiae while allowing broader flaws to persist. I'd chalk that up to a lack of design vision to guide the testing.
I think there was a clear line drawn at least with the G+ integration, since they started consolidating the appearance of all services under the same UI and same color codes. There is a now a clear "design vision" in Google (even if it clearly sucks bad) and it's clearly not as Engineers-driven/Fonctionality-driven as before. Having Icons without text next to it is not just a style issue: it's a well known flaw in terms of usability. Everyone knows icons+text are superior to icons only. Especially when your icons are dark grey on light grey background. No differentiation whatsoever. I can't believe someone was actually paid to do this.
At least, we can see there was a significant increase of "Gmail interface sucks" articles when the new UI came out. That has to represent some part of the users, to the least.
The problem is that the numbers don't matter in isolation. It's wrapped in two assumptions:
* That it's possible to make something that the vast majority of the population likes. Especially for something as complex and context dependent as email. No matter what Google does some section of the population won't like it.
* That it matters to Google (or any other company) that N% hate a product. I don't care if 75% of the world hates my product if the remaining 25% love it - and shovel money in my direction because of it.