By now, many of us have app and content purchases tied to those accounts, and migrating from Gmail/Drive/Photos isn't practical, so we're stuck getting new Google features later or never.
The only good way to use a custom domain with Google is to pay. Clearly a lot of us are happy doing so. I know I pay around £200 per year to Google for my one account, the storage and media subscriptions (music, magazines, etc)... But for this I get incomplete features (Now) and locked out of some services (Play Family).
I use (paid) Google Apps for personal & family stuff - and this seems like a pretty arbitrary restriction on Play Music.
Anecdotally - I was a longtime Rdio user, from beta until they recently closed. I did the trial of Google Play Music and it's not only the best good product I've ever used, but I have already gifted 6 months to my sister. Christmas is coming up, and I'm tempted to gift it again to other people.
If you enjoy music and haven't checked out Google Play Music, I'd encourage you to give it a shot. So far it's everything I've wanted plus more!