A downwards user share trajectory, but an upwards technical one as of late. But yeah, it's pretty worrying that a webkit monoculture is developing. At least webkit is open source, so google won't really have a true monopoly (at least apple and microsoft will have competing (if still webkit based) implementations).
A downwards user-share trajectory, and a downwards reputation trajectory. I don't think that all the hate Mozilla's getting is deserved, but a lot of people didn't like that one easter egg, or the mess with Pocket.
The Webkit/Blink engine is far from being single vendor, they are vibrant open source projects with many companies that contribute to the development efforts, pretty much all browsers except for Firefox will be based on it soon.
Web developers would rather have a single good engine, assuming that it's good enough and continuously developed.
IE failed back when they all but ceased development for a number of years and since it was closed source nobody could take it over.
Blink is a WebKit fork and is used by everyone, from Chrome to Opera, and soon, Edge. Apple and Google have gone their separate ways, and will make decisions separately, so WebKit isn't really contributing to the Blink monopoly.