There's a few issues with this though. Take various animations such as the one on the toolbar that some Google apps have (the arrow that spins into a hamburger and back again). Easy to do if you're just loading different fragments into a Framelayout, impossible to do with activities as you reload the entire toolbar on an activity change.
It's a general problem in the Android world. Some things can only be done as an activity whilst others as fragments.
In fact this is a problem even on Googles own apps. Right now if i tap things in the nav panel of the Gmail app it's a crap shoot whether i open a fragment or activity. Things like inbox/sent are loaded as fragments. Things like settings are loaded as activities with a completely different loading in animation. Help is different again. Lots of different ways of loading in a view all coming from options in the same menu. It's pretty fucked up form a UI perspective.