Because I'm a geek I did a graph of used GTI prices when I was buying mine:

Very roughly depreciation in the first 5-6 years seems to be about £2k/year and then ease back to about £1.5k per year. Mk5's seem to have bottomed out at between £5k-£7k for good ones and are starting to rise in price again with all the recent articles about them being a bargain.
The Mk7 was so well reviewed, and so far the engine seems solid, so it seems like another classic in the making to me.. but.. and it's a big but, in three years time the electric revolution will be in full swing and battery ranges will probably be 500 miles plus by then. That's only going to do bad things to the used prices for cars with an IC engine in my opinion. Add on the arrival of self-drive cars in 3-5 years time and that will probably further deplete the demand for used cars from this era.
I weighed all that up and still bought a 2015 GTI and couldn't be more pleased with my decision

It does make leasing a tempting option at the moment though.