I've kept plenty of car way past the expiry of their warranties and never given it a thought. Sure, I've had things go wrong with one or two, but that's to be expected. I reckoned if it cost me £1000 a year in servicing and repairs, that was far cheaper than buying a new car and suffering the depreciation all over again.
Considering that your GTI sounds like a well looked after example, I'd be tempted to enjoy it for a while longer. In another couple of years, technology will have moved on further and you may well be able to buy into that, rather than a getting to the end of the line model Golf if you bought one now.