I think that cups fall under the "aspirational" car category. By that I mean, that a lot of people read magazine articles about the cup being "hard, a drivers car, a track day car, etc, etc" and fancy themselves as fitting that particular image. Thumbs up to Renault for having achieved that. The cup is probably the cheapest hatch you can buy that will give you that instant "racer" street cred. As you rightly say, the 172 is more expensive. So I imagine, there are loads of aspiring young/budget/mean wannabe racers out there buying cups, and youre bound to see some of them drop out after a while. Like most other things in life, driving around in a cup is a compromise, and when its hot, bumper to bumper traffic, potholes etc, some people may not be willing to compromise. Eventually the market will stabilise and the cups will mostly end up with dedicated petrolheads like me
personally, I chose a cup fully aware of the compromises I had to "endure" (read enjoy), and if anything Ive been accentuating some of those compromises! For my own needs (town driving + regular track use + financial ability to write it off) the cup is just perfect. If I didnt have occasional need to drop the kids at school, Id be driving a caterham.
Voila, thats my 2 cents..