That was the final nail in the coffin. The reason why they didn't hit adoption in the first place is because Google prohibited their application on MS devices. Mobile YouTube already wasn't good enough, and without the rest of the GSuite (Maps, Gmail, Chrome, Calendar, Translate) it was dead in the water. And no, HERE maps and third-party clients were not good enough to tip the scale.
This wasn't (only) about Google refusing to make apps for the WP, it was Google actively preventing WP apps from accessing their services where they could. Microsoft made a very nice YouTube client, for example, and Google simply denied YT access if they detected you were using it.