With all due respect, the people that already own a Mustang are only part of the Ford's target demographics with the car. Since its inception, it's always been a personalizable car, and with that comes the variety of engine sizes and options (color scheme, interior, etc). On top of that, the Mustang has always thrived on a large portion of their buying audience being new (ie: college kids, first time drivers that want something sporty), and the soccer moms and women that really don't give a damn about anything other than the fact that it is a Mustang, it's sporty, has good get up and go, and makes them feel young again.
In other words, Ford is selling them the overall mojo and reputation that comes with a Mustang, and the overall aura and excitement that it gives people is good enough for most buyers. Not everyone is as picky as many of us.
At work last week, there was an old grandma in her late 60's that had pulled up in a red Mustang. I said, "nice Stang!". She said it would be time to be getting a new one soon. The vast majority of Mustang owners just own one because it's a fun car, and in the end, it's pretty hard to argue with that.
1999 35th Anniversary V6 (FRPP 4.10's, Auburn LSD, MAC catback dual exhaust w/h-pipe, P&P'd upper and lower intakes, 70mm Accufab TB with intake port matched/ polished, BBK ceramic shorties, etc)
1991 T-Bird Super Coupe 3.8 Eaton M90 intercooled 12 psi, factory 8.8 lsd (double intercooler, etc)