If you think people won't buy the "8" in droves, you're really not connected to reality. The 7s will be the 4th year of the "6" form factor, and many, many people will want something different as the new one.
Questions of my connection to reality aside, yes, many people will buy the "8" - not nearly as many as will buy the 7s, because availability will be
much more limited, and the price will be
much higher. The percentage of readers of Apple rumors websites who go for the "8" will skew quite a bit higher than the general population, many of whom will say, "oh, that looks cool - six week wait? - $1300? - GTFO!".
I don't expect Apple has the parts availability to build the "8" at the rates they've built their previous models - that's part of the plan: for years, Apple has been limited in phone design by what they can build at a rate of nearly half a million every day, to sell at essentially one price point, in terms of both cost and parts availability. They're finally expanding the lineup to have a separate flagship model that can use parts they can't get in quantity half-million-a-day, and that they can sell for
much more than the standard model (thus allowing for the cost of all those fancy parts). We don't really have a standardized definition of what quantity "droves" represents, but, yes, they'll sell a lot of "8"s. But the
large majority of phones they sell in the next year will be the 7s/7s+. Anyway, your original concern was your mother not being able to figure out the virtual home button, and my answer stands: no problem, have her get a 7s rather than trying to get an "8" - if she really can't understand a virtual home button (she might surprise you), chances are the other fancy features of the "8" will be lost on her anyway (or at least not worth the many hundreds of dollars extra it'll cost).
My other point stands also - many people are debating the implementation/sanity of features on the "8" as though it's the only phone Apple will be releasing. The "8" will not be the
main iPhone being sold over the next year. It'll get huge mindshare/press, but it won't sell as well as the 7s.
FWIW, I'm looking forward to the 7s to replace my 6 - it'll look basically the same (don't care), but with everything vastly improved (way faster everything, more memory, better camera, better screen, more water resistance, etc.).