Google is/was quite successful before they got design (let's say success in this context means something like: a large audience).
And even when they decided to get design: it took them a relatively short amount of time to get things "right".
I agree Material Design looks very nice. But I think functional value (just picking a term here) is so much more important, by an order of magnitude.
Speed, functional value (as you said), simplicity, and such really ought to be considered part of design just as much as a visually appealing set of colors on an interface is.
With the new support lib (22.2), Google has released a design lib which implements some material elements. With this common base, I hope to see the design of the different first party apps harmonize a little bit more in the coming months.
Also, while I absolutely hate even firing up Maps (but I need to), I find that people, whose first experience with them is the new interface, like it very much.
Google did well before they got design, but they have probably widened their audience and increased conversion by getting design so increasing their profitability.
Aesthetically pleasing design is neither necessary nor sufficient to be a successful, but it definitely helps.
When design is about cramming some animation in just about everything you can show on the screen, and about how fancy your colors are going to be – yes, I agree, it's relatively unimportant.
But real design is not about that at all. In fact, functionality has been part of the core debate in design, for at least a century now. Ask any designer – not only the ones dealing with UI, and they'll tell you.