I wrote this essay of why it was struggling in March of this year.
I'm not sure I see how a habit and frequency of use aren't synonymous with each other? I think the two axis should be perceived (or actual) utility, and seamlessness (inverse of attention required). If something is incredibly useful, but requires an infinite amount of attention, nobody would use it, and if something is completely seamless to use, but has no utility, still nobody will use it. You need a combination of the two.