> They rebranded the exact same code base with largely the same staff. I don't know why you perceive this to have been a break.

This is actually completely false. (Source: I worked at Google at the time this transition was made, though not on GPM or YTM).

Two completely different codebases and teams and in fact YTM launched missing a boatload of features that Play Music had. Including annoyingly the ability to use our own Chromecast features on the desktop/browser version. (I worked on the Cast team). There were quite a few angry internal buganizer tickets.