It's a long time in coming, notwithstanding that regulations and FCC compliance seem to be the biggest regulatory obstacles preventing this idea from becoming reality earlier on during the evolution of mobile devices.
The only thing missing is that I had to set up my launcher, install all the apps again, configure my IMAP email client, and configure some third-party apps that didn't sync notification sounds.
[1]: In case you're not familiar, it added the ability to make and receive VoIP calls.
I suspect there's always been a strong element of competition based on physical size in computing. PC cases are about as small as you can take that and still leave room for customisability.
So my guess is the overlap between the competitive hardware concept space, the custom-build concept space, and the pocket device concept space might be big enough to drive a tiny niche, but not a full consumer frenzy.