As opposed to what? An Opel?
HA HA HA HA HA HA.
Well, lets put it this way. I bought my OPC+ new in Oct 2006 for R278 000 all in. I sold it to N1 City dealership in Oct last year for R100 000 on the dot. I knew they were raping me, but I didn't know how much until I found out that they sold it a week or two later for R159 000. Still, after 8 years I got 36% back. If I sold it privately for say R150k it would be 54% after 8 years.
Basically all dealers will do this once a car is out of motorplan/service plan. CPA they claim.
I plan to do exactly that.
Well probably not 15 years, but definitely 10 years at least.
I bought it for R199 000 as it was just over 3 years old (2008 model at the end of 2011) and it's worth about R160 000 now.
Will probably still fetch R100k in 2018 when it's 10 years old which means a 50% loss in total or 5% per year which isn't really all that bad.
.
Having a quick look online for your 2008 gti, the max is R155 000 (retail) and I don't think they are selling. If you tried to sell to a dealer i would be surprised if you got R100k. So I think you are being a bit optimistic in your assessment.